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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
ba318903 | 3 | @c Copyright (C) 1990-1995, 1998-2014 Free Software Foundation, Inc. |
b8d4c8d0 | 4 | @c See the file elisp.texi for copying conditions. |
ecc6530d | 5 | @node Variables |
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6 | @chapter Variables |
7 | @cindex variable | |
8 | ||
9 | A @dfn{variable} is a name used in a program to stand for a value. | |
1021c761 | 10 | In Lisp, each variable is represented by a Lisp symbol |
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11 | (@pxref{Symbols}). The variable name is simply the symbol's name, and |
12 | the variable's value is stored in the symbol's value cell@footnote{To | |
362397ed | 13 | be precise, under the default @dfn{dynamic scoping} rule, the value |
735cc5ca | 14 | cell always holds the variable's current value, but this is not the |
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15 | case under the @dfn{lexical scoping} rule. @xref{Variable Scoping}, |
16 | for details.}. @xref{Symbol Components}. In Emacs Lisp, the use of a | |
735cc5ca | 17 | symbol as a variable is independent of its use as a function name. |
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18 | |
19 | As previously noted in this manual, a Lisp program is represented | |
20 | primarily by Lisp objects, and only secondarily as text. The textual | |
21 | form of a Lisp program is given by the read syntax of the Lisp objects | |
22 | that constitute the program. Hence, the textual form of a variable in | |
23 | a Lisp program is written using the read syntax for the symbol | |
32770114 | 24 | representing the variable. |
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25 | |
26 | @menu | |
d032d5e7 SM |
27 | * Global Variables:: Variable values that exist permanently, everywhere. |
28 | * Constant Variables:: Certain "variables" have values that never change. | |
29 | * Local Variables:: Variable values that exist only temporarily. | |
30 | * Void Variables:: Symbols that lack values. | |
31 | * Defining Variables:: A definition says a symbol is used as a variable. | |
32 | * Tips for Defining:: Things you should think about when you | |
b8d4c8d0 | 33 | define a variable. |
d032d5e7 | 34 | * Accessing Variables:: Examining values of variables whose names |
b8d4c8d0 | 35 | are known only at run time. |
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36 | * Setting Variables:: Storing new values in variables. |
37 | * Variable Scoping:: How Lisp chooses among local and global values. | |
38 | * Buffer-Local Variables:: Variable values in effect only in one buffer. | |
39 | * File Local Variables:: Handling local variable lists in files. | |
40 | * Directory Local Variables:: Local variables common to all files in a directory. | |
d032d5e7 | 41 | * Variable Aliases:: Variables that are aliases for other variables. |
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42 | * Variables with Restricted Values:: Non-constant variables whose value can |
43 | @emph{not} be an arbitrary Lisp object. | |
5887564d | 44 | * Generalized Variables:: Extending the concept of variables. |
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45 | @end menu |
46 | ||
47 | @node Global Variables | |
48 | @section Global Variables | |
49 | @cindex global variable | |
50 | ||
51 | The simplest way to use a variable is @dfn{globally}. This means that | |
52 | the variable has just one value at a time, and this value is in effect | |
53 | (at least for the moment) throughout the Lisp system. The value remains | |
54 | in effect until you specify a new one. When a new value replaces the | |
55 | old one, no trace of the old value remains in the variable. | |
56 | ||
57 | You specify a value for a symbol with @code{setq}. For example, | |
58 | ||
59 | @example | |
60 | (setq x '(a b)) | |
61 | @end example | |
62 | ||
63 | @noindent | |
64 | gives the variable @code{x} the value @code{(a b)}. Note that | |
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65 | @code{setq} is a special form (@pxref{Special Forms}); it does not |
66 | evaluate its first argument, the name of the variable, but it does | |
67 | evaluate the second argument, the new value. | |
b8d4c8d0 | 68 | |
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69 | Once the variable has a value, you can refer to it by using the |
70 | symbol itself as an expression. Thus, | |
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71 | |
72 | @example | |
73 | @group | |
74 | x @result{} (a b) | |
75 | @end group | |
76 | @end example | |
77 | ||
78 | @noindent | |
79 | assuming the @code{setq} form shown above has already been executed. | |
80 | ||
81 | If you do set the same variable again, the new value replaces the old | |
82 | one: | |
83 | ||
84 | @example | |
85 | @group | |
86 | x | |
87 | @result{} (a b) | |
88 | @end group | |
89 | @group | |
90 | (setq x 4) | |
91 | @result{} 4 | |
92 | @end group | |
93 | @group | |
94 | x | |
95 | @result{} 4 | |
96 | @end group | |
97 | @end example | |
98 | ||
99 | @node Constant Variables | |
100 | @section Variables that Never Change | |
7018dbe7 | 101 | @cindex @code{setting-constant} error |
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102 | @cindex keyword symbol |
103 | @cindex variable with constant value | |
104 | @cindex constant variables | |
105 | @cindex symbol that evaluates to itself | |
106 | @cindex symbol with constant value | |
107 | ||
108 | In Emacs Lisp, certain symbols normally evaluate to themselves. These | |
109 | include @code{nil} and @code{t}, as well as any symbol whose name starts | |
110 | with @samp{:} (these are called @dfn{keywords}). These symbols cannot | |
111 | be rebound, nor can their values be changed. Any attempt to set or bind | |
112 | @code{nil} or @code{t} signals a @code{setting-constant} error. The | |
113 | same is true for a keyword (a symbol whose name starts with @samp{:}), | |
114 | if it is interned in the standard obarray, except that setting such a | |
115 | symbol to itself is not an error. | |
116 | ||
117 | @example | |
118 | @group | |
119 | nil @equiv{} 'nil | |
120 | @result{} nil | |
121 | @end group | |
122 | @group | |
123 | (setq nil 500) | |
124 | @error{} Attempt to set constant symbol: nil | |
125 | @end group | |
126 | @end example | |
127 | ||
128 | @defun keywordp object | |
129 | function returns @code{t} if @var{object} is a symbol whose name | |
130 | starts with @samp{:}, interned in the standard obarray, and returns | |
131 | @code{nil} otherwise. | |
132 | @end defun | |
133 | ||
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134 | These constants are fundamentally different from the ``constants'' |
135 | defined using the @code{defconst} special form (@pxref{Defining | |
136 | Variables}). A @code{defconst} form serves to inform human readers | |
137 | that you do not intend to change the value of a variable, but Emacs | |
138 | does not raise an error if you actually change it. | |
139 | ||
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140 | @node Local Variables |
141 | @section Local Variables | |
142 | @cindex binding local variables | |
143 | @cindex local variables | |
144 | @cindex local binding | |
145 | @cindex global binding | |
146 | ||
147 | Global variables have values that last until explicitly superseded | |
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148 | with new values. Sometimes it is useful to give a variable a |
149 | @dfn{local value}---a value that takes effect only within a certain | |
150 | part of a Lisp program. When a variable has a local value, we say | |
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151 | that it is @dfn{locally bound} to that value, and that it is a |
152 | @dfn{local variable}. | |
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153 | |
154 | For example, when a function is called, its argument variables | |
155 | receive local values, which are the actual arguments supplied to the | |
156 | function call; these local bindings take effect within the body of the | |
157 | function. To take another example, the @code{let} special form | |
158 | explicitly establishes local bindings for specific variables, which | |
159 | take effect within the body of the @code{let} form. | |
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160 | |
161 | We also speak of the @dfn{global binding}, which is where | |
162 | (conceptually) the global value is kept. | |
163 | ||
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164 | @cindex shadowing of variables |
165 | Establishing a local binding saves away the variable's previous | |
166 | value (or lack of one). We say that the previous value is | |
167 | @dfn{shadowed}. Both global and local values may be shadowed. If a | |
168 | local binding is in effect, using @code{setq} on the local variable | |
169 | stores the specified value in the local binding. When that local | |
170 | binding is no longer in effect, the previously shadowed value (or lack | |
171 | of one) comes back. | |
172 | ||
b8d4c8d0 | 173 | @cindex current binding |
1df7defd | 174 | A variable can have more than one local binding at a time (e.g., if |
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175 | there are nested @code{let} forms that bind the variable). The |
176 | @dfn{current binding} is the local binding that is actually in effect. | |
177 | It determines the value returned by evaluating the variable symbol, | |
178 | and it is the binding acted on by @code{setq}. | |
179 | ||
180 | For most purposes, you can think of the current binding as the | |
181 | ``innermost'' local binding, or the global binding if there is no | |
182 | local binding. To be more precise, a rule called the @dfn{scoping | |
183 | rule} determines where in a program a local binding takes effect. The | |
184 | default scoping rule in Emacs Lisp is called @dfn{dynamic scoping}, | |
185 | which simply states that the current binding at any given point in the | |
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186 | execution of a program is the most recently-created binding for that |
187 | variable that still exists. For details about dynamic scoping, and an | |
188 | alternative scoping rule called @dfn{lexical scoping}, @xref{Variable | |
189 | Scoping}. | |
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190 | |
191 | The special forms @code{let} and @code{let*} exist to create local | |
192 | bindings: | |
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193 | |
194 | @defspec let (bindings@dots{}) forms@dots{} | |
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195 | This special form sets up local bindings for a certain set of |
196 | variables, as specified by @var{bindings}, and then evaluates all of | |
197 | the @var{forms} in textual order. Its return value is the value of | |
198 | the last form in @var{forms}. | |
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199 | |
200 | Each of the @var{bindings} is either @w{(i) a} symbol, in which case | |
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201 | that symbol is locally bound to @code{nil}; or @w{(ii) a} list of the |
202 | form @code{(@var{symbol} @var{value-form})}, in which case | |
203 | @var{symbol} is locally bound to the result of evaluating | |
204 | @var{value-form}. If @var{value-form} is omitted, @code{nil} is used. | |
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205 | |
206 | All of the @var{value-form}s in @var{bindings} are evaluated in the | |
207 | order they appear and @emph{before} binding any of the symbols to them. | |
208 | Here is an example of this: @code{z} is bound to the old value of | |
209 | @code{y}, which is 2, not the new value of @code{y}, which is 1. | |
210 | ||
211 | @example | |
212 | @group | |
213 | (setq y 2) | |
214 | @result{} 2 | |
215 | @end group | |
1021c761 | 216 | |
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217 | @group |
218 | (let ((y 1) | |
219 | (z y)) | |
220 | (list y z)) | |
221 | @result{} (1 2) | |
222 | @end group | |
223 | @end example | |
224 | @end defspec | |
225 | ||
226 | @defspec let* (bindings@dots{}) forms@dots{} | |
227 | This special form is like @code{let}, but it binds each variable right | |
228 | after computing its local value, before computing the local value for | |
229 | the next variable. Therefore, an expression in @var{bindings} can | |
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230 | refer to the preceding symbols bound in this @code{let*} form. |
231 | Compare the following example with the example above for @code{let}. | |
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232 | |
233 | @example | |
234 | @group | |
235 | (setq y 2) | |
236 | @result{} 2 | |
237 | @end group | |
1021c761 | 238 | |
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239 | @group |
240 | (let* ((y 1) | |
241 | (z y)) ; @r{Use the just-established value of @code{y}.} | |
242 | (list y z)) | |
243 | @result{} (1 1) | |
244 | @end group | |
245 | @end example | |
246 | @end defspec | |
247 | ||
248 | Here is a complete list of the other facilities that create local | |
249 | bindings: | |
250 | ||
251 | @itemize @bullet | |
252 | @item | |
253 | Function calls (@pxref{Functions}). | |
254 | ||
255 | @item | |
256 | Macro calls (@pxref{Macros}). | |
257 | ||
258 | @item | |
259 | @code{condition-case} (@pxref{Errors}). | |
260 | @end itemize | |
261 | ||
262 | Variables can also have buffer-local bindings (@pxref{Buffer-Local | |
e388c68f | 263 | Variables}); a few variables have terminal-local bindings |
3ec61d4e | 264 | (@pxref{Multiple Terminals}). These kinds of bindings work somewhat |
e388c68f | 265 | like ordinary local bindings, but they are localized depending on |
1021c761 | 266 | ``where'' you are in Emacs. |
b8d4c8d0 | 267 | |
01f17ae2 | 268 | @defopt max-specpdl-size |
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269 | @anchor{Definition of max-specpdl-size} |
270 | @cindex variable limit error | |
271 | @cindex evaluation error | |
272 | @cindex infinite recursion | |
273 | This variable defines the limit on the total number of local variable | |
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274 | bindings and @code{unwind-protect} cleanups (see @ref{Cleanups,, |
275 | Cleaning Up from Nonlocal Exits}) that are allowed before Emacs | |
276 | signals an error (with data @code{"Variable binding depth exceeds | |
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277 | max-specpdl-size"}). |
278 | ||
279 | This limit, with the associated error when it is exceeded, is one way | |
280 | that Lisp avoids infinite recursion on an ill-defined function. | |
281 | @code{max-lisp-eval-depth} provides another limit on depth of nesting. | |
282 | @xref{Definition of max-lisp-eval-depth,, Eval}. | |
283 | ||
1021c761 | 284 | The default value is 1300. Entry to the Lisp debugger increases the |
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285 | value, if there is little room left, to make sure the debugger itself |
286 | has room to execute. | |
01f17ae2 | 287 | @end defopt |
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288 | |
289 | @node Void Variables | |
290 | @section When a Variable is ``Void'' | |
7018dbe7 | 291 | @cindex @code{void-variable} error |
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292 | @cindex void variable |
293 | ||
735cc5ca | 294 | We say that a variable is void if its symbol has an unassigned value |
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295 | cell (@pxref{Symbol Components}). |
296 | ||
297 | Under Emacs Lisp's default dynamic scoping rule (@pxref{Variable | |
298 | Scoping}), the value cell stores the variable's current (local or | |
299 | global) value. Note that an unassigned value cell is @emph{not} the | |
300 | same as having @code{nil} in the value cell. The symbol @code{nil} is | |
301 | a Lisp object and can be the value of a variable, just as any other | |
302 | object can be; but it is still a value. If a variable is void, trying | |
303 | to evaluate the variable signals a @code{void-variable} error, instead | |
304 | of returning a value. | |
305 | ||
306 | Under the optional lexical scoping rule, the value cell only holds | |
307 | the variable's global value---the value outside of any lexical binding | |
308 | construct. When a variable is lexically bound, the local value is | |
309 | determined by the lexical environment; hence, variables can have local | |
310 | values even if their symbols' value cells are unassigned. | |
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311 | |
312 | @defun makunbound symbol | |
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313 | This function empties out the value cell of @var{symbol}, making the |
314 | variable void. It returns @var{symbol}. | |
b8d4c8d0 | 315 | |
735cc5ca | 316 | If @var{symbol} has a dynamic local binding, @code{makunbound} voids |
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317 | the current binding, and this voidness lasts only as long as the local |
318 | binding is in effect. Afterwards, the previously shadowed local or | |
319 | global binding is reexposed; then the variable will no longer be void, | |
320 | unless the reexposed binding is void too. | |
b8d4c8d0 | 321 | |
1021c761 | 322 | Here are some examples (assuming dynamic binding is in effect): |
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323 | |
324 | @smallexample | |
325 | @group | |
326 | (setq x 1) ; @r{Put a value in the global binding.} | |
327 | @result{} 1 | |
328 | (let ((x 2)) ; @r{Locally bind it.} | |
329 | (makunbound 'x) ; @r{Void the local binding.} | |
330 | x) | |
331 | @error{} Symbol's value as variable is void: x | |
332 | @end group | |
333 | @group | |
334 | x ; @r{The global binding is unchanged.} | |
335 | @result{} 1 | |
336 | ||
337 | (let ((x 2)) ; @r{Locally bind it.} | |
338 | (let ((x 3)) ; @r{And again.} | |
339 | (makunbound 'x) ; @r{Void the innermost-local binding.} | |
340 | x)) ; @r{And refer: it's void.} | |
341 | @error{} Symbol's value as variable is void: x | |
342 | @end group | |
343 | ||
344 | @group | |
345 | (let ((x 2)) | |
346 | (let ((x 3)) | |
347 | (makunbound 'x)) ; @r{Void inner binding, then remove it.} | |
348 | x) ; @r{Now outer @code{let} binding is visible.} | |
349 | @result{} 2 | |
350 | @end group | |
351 | @end smallexample | |
352 | @end defun | |
353 | ||
b8d4c8d0 | 354 | @defun boundp variable |
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355 | This function returns @code{t} if @var{variable} (a symbol) is not |
356 | void, and @code{nil} if it is void. | |
357 | ||
358 | Here are some examples (assuming dynamic binding is in effect): | |
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359 | |
360 | @smallexample | |
361 | @group | |
362 | (boundp 'abracadabra) ; @r{Starts out void.} | |
363 | @result{} nil | |
364 | @end group | |
365 | @group | |
366 | (let ((abracadabra 5)) ; @r{Locally bind it.} | |
367 | (boundp 'abracadabra)) | |
368 | @result{} t | |
369 | @end group | |
370 | @group | |
371 | (boundp 'abracadabra) ; @r{Still globally void.} | |
372 | @result{} nil | |
373 | @end group | |
374 | @group | |
375 | (setq abracadabra 5) ; @r{Make it globally nonvoid.} | |
376 | @result{} 5 | |
377 | @end group | |
378 | @group | |
379 | (boundp 'abracadabra) | |
380 | @result{} t | |
381 | @end group | |
382 | @end smallexample | |
383 | @end defun | |
384 | ||
385 | @node Defining Variables | |
386 | @section Defining Global Variables | |
387 | @cindex variable definition | |
388 | ||
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389 | A @dfn{variable definition} is a construct that announces your |
390 | intention to use a symbol as a global variable. It uses the special | |
391 | forms @code{defvar} or @code{defconst}, which are documented below. | |
392 | ||
393 | A variable definition serves three purposes. First, it informs | |
394 | people who read the code that the symbol is @emph{intended} to be used | |
395 | a certain way (as a variable). Second, it informs the Lisp system of | |
396 | this, optionally supplying an initial value and a documentation | |
397 | string. Third, it provides information to programming tools such as | |
398 | @command{etags}, allowing them to find where the variable was defined. | |
399 | ||
400 | The difference between @code{defconst} and @code{defvar} is mainly a | |
401 | matter of intent, serving to inform human readers of whether the value | |
402 | should ever change. Emacs Lisp does not actually prevent you from | |
403 | changing the value of a variable defined with @code{defconst}. One | |
404 | notable difference between the two forms is that @code{defconst} | |
405 | unconditionally initializes the variable, whereas @code{defvar} | |
406 | initializes it only if it is originally void. | |
407 | ||
408 | To define a customizable variable, you should use @code{defcustom} | |
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409 | (which calls @code{defvar} as a subroutine). @xref{Variable |
410 | Definitions}. | |
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411 | |
412 | @defspec defvar symbol [value [doc-string]] | |
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413 | This special form defines @var{symbol} as a variable. Note that |
414 | @var{symbol} is not evaluated; the symbol to be defined should appear | |
415 | explicitly in the @code{defvar} form. The variable is marked as | |
416 | @dfn{special}, meaning that it should always be dynamically bound | |
417 | (@pxref{Variable Scoping}). | |
b8d4c8d0 | 418 | |
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419 | If @var{value} is specified, and @var{symbol} is void (i.e., it has no |
420 | dynamically bound value; @pxref{Void Variables}), then @var{value} is | |
421 | evaluated and @var{symbol} is set to the result. But if @var{symbol} | |
422 | is not void, @var{value} is not evaluated, and @var{symbol}'s value is | |
423 | left unchanged. If @var{value} is omitted, the value of @var{symbol} | |
424 | is not changed in any case. | |
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425 | |
426 | If @var{symbol} has a buffer-local binding in the current buffer, | |
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427 | @code{defvar} acts on the default value, which is buffer-independent, |
428 | rather than the buffer-local binding. It sets the default value if | |
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429 | the default value is void. @xref{Buffer-Local Variables}. |
430 | ||
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431 | If @var{symbol} is already lexically bound (e.g., if the @code{defvar} |
432 | form occurs in a @code{let} form with lexical binding enabled), then | |
433 | @code{defvar} sets the dynamic value. The lexical binding remains in | |
434 | effect until its binding construct exits. @xref{Variable Scoping}. | |
435 | ||
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436 | When you evaluate a top-level @code{defvar} form with @kbd{C-M-x} in |
437 | Emacs Lisp mode (@code{eval-defun}), a special feature of | |
438 | @code{eval-defun} arranges to set the variable unconditionally, without | |
439 | testing whether its value is void. | |
440 | ||
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441 | If the @var{doc-string} argument is supplied, it specifies the |
442 | documentation string for the variable (stored in the symbol's | |
443 | @code{variable-documentation} property). @xref{Documentation}. | |
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444 | |
445 | Here are some examples. This form defines @code{foo} but does not | |
446 | initialize it: | |
447 | ||
448 | @example | |
449 | @group | |
450 | (defvar foo) | |
451 | @result{} foo | |
452 | @end group | |
453 | @end example | |
454 | ||
455 | This example initializes the value of @code{bar} to @code{23}, and gives | |
456 | it a documentation string: | |
457 | ||
458 | @example | |
459 | @group | |
460 | (defvar bar 23 | |
461 | "The normal weight of a bar.") | |
462 | @result{} bar | |
463 | @end group | |
464 | @end example | |
465 | ||
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466 | The @code{defvar} form returns @var{symbol}, but it is normally used |
467 | at top level in a file where its value does not matter. | |
468 | @end defspec | |
469 | ||
2640fa86 | 470 | @cindex constant variables |
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471 | @defspec defconst symbol value [doc-string] |
472 | This special form defines @var{symbol} as a value and initializes it. | |
473 | It informs a person reading your code that @var{symbol} has a standard | |
474 | global value, established here, that should not be changed by the user | |
475 | or by other programs. Note that @var{symbol} is not evaluated; the | |
476 | symbol to be defined must appear explicitly in the @code{defconst}. | |
477 | ||
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478 | The @code{defconst} form, like @code{defvar}, marks the variable as |
479 | @dfn{special}, meaning that it should always be dynamically bound | |
480 | (@pxref{Variable Scoping}). In addition, it marks the variable as | |
481 | risky (@pxref{File Local Variables}). | |
482 | ||
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483 | @code{defconst} always evaluates @var{value}, and sets the value of |
484 | @var{symbol} to the result. If @var{symbol} does have a buffer-local | |
485 | binding in the current buffer, @code{defconst} sets the default value, | |
486 | not the buffer-local value. (But you should not be making | |
487 | buffer-local bindings for a symbol that is defined with | |
488 | @code{defconst}.) | |
489 | ||
44e97401 | 490 | An example of the use of @code{defconst} is Emacs's definition of |
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491 | @code{float-pi}---the mathematical constant @math{pi}, which ought not |
492 | to be changed by anyone (attempts by the Indiana State Legislature | |
493 | notwithstanding). As the second form illustrates, however, | |
494 | @code{defconst} is only advisory. | |
b8d4c8d0 GM |
495 | |
496 | @example | |
497 | @group | |
ec8a6295 CY |
498 | (defconst float-pi 3.141592653589793 "The value of Pi.") |
499 | @result{} float-pi | |
b8d4c8d0 GM |
500 | @end group |
501 | @group | |
ec8a6295 CY |
502 | (setq float-pi 3) |
503 | @result{} float-pi | |
b8d4c8d0 GM |
504 | @end group |
505 | @group | |
ec8a6295 | 506 | float-pi |
b8d4c8d0 GM |
507 | @result{} 3 |
508 | @end group | |
509 | @end example | |
510 | @end defspec | |
511 | ||
1021c761 CY |
512 | @strong{Warning:} If you use a @code{defconst} or @code{defvar} |
513 | special form while the variable has a local binding (made with | |
514 | @code{let}, or a function argument), it sets the local binding rather | |
515 | than the global binding. This is not what you usually want. To | |
516 | prevent this, use these special forms at top level in a file, where | |
517 | normally no local binding is in effect, and make sure to load the file | |
518 | before making a local binding for the variable. | |
b8d4c8d0 GM |
519 | |
520 | @node Tips for Defining | |
521 | @section Tips for Defining Variables Robustly | |
522 | ||
523 | When you define a variable whose value is a function, or a list of | |
524 | functions, use a name that ends in @samp{-function} or | |
525 | @samp{-functions}, respectively. | |
526 | ||
527 | There are several other variable name conventions; | |
528 | here is a complete list: | |
529 | ||
530 | @table @samp | |
531 | @item @dots{}-hook | |
532 | The variable is a normal hook (@pxref{Hooks}). | |
533 | ||
534 | @item @dots{}-function | |
535 | The value is a function. | |
536 | ||
537 | @item @dots{}-functions | |
538 | The value is a list of functions. | |
539 | ||
540 | @item @dots{}-form | |
541 | The value is a form (an expression). | |
542 | ||
543 | @item @dots{}-forms | |
544 | The value is a list of forms (expressions). | |
545 | ||
546 | @item @dots{}-predicate | |
547 | The value is a predicate---a function of one argument that returns | |
548 | non-@code{nil} for ``good'' arguments and @code{nil} for ``bad'' | |
549 | arguments. | |
550 | ||
551 | @item @dots{}-flag | |
552 | The value is significant only as to whether it is @code{nil} or not. | |
cc5a5e2d | 553 | Since such variables often end up acquiring more values over time, |
0befcaca | 554 | this convention is not strongly recommended. |
b8d4c8d0 GM |
555 | |
556 | @item @dots{}-program | |
557 | The value is a program name. | |
558 | ||
559 | @item @dots{}-command | |
560 | The value is a whole shell command. | |
561 | ||
562 | @item @dots{}-switches | |
563 | The value specifies options for a command. | |
564 | @end table | |
565 | ||
566 | When you define a variable, always consider whether you should mark | |
32770114 | 567 | it as ``safe'' or ``risky''; see @ref{File Local Variables}. |
b8d4c8d0 GM |
568 | |
569 | When defining and initializing a variable that holds a complicated | |
570 | value (such as a keymap with bindings in it), it's best to put the | |
571 | entire computation of the value into the @code{defvar}, like this: | |
572 | ||
573 | @example | |
574 | (defvar my-mode-map | |
575 | (let ((map (make-sparse-keymap))) | |
576 | (define-key map "\C-c\C-a" 'my-command) | |
577 | @dots{} | |
578 | map) | |
579 | @var{docstring}) | |
580 | @end example | |
581 | ||
582 | @noindent | |
583 | This method has several benefits. First, if the user quits while | |
584 | loading the file, the variable is either still uninitialized or | |
585 | initialized properly, never in-between. If it is still uninitialized, | |
586 | reloading the file will initialize it properly. Second, reloading the | |
587 | file once the variable is initialized will not alter it; that is | |
1021c761 CY |
588 | important if the user has run hooks to alter part of the contents |
589 | (such as, to rebind keys). Third, evaluating the @code{defvar} form | |
590 | with @kbd{C-M-x} will reinitialize the map completely. | |
b8d4c8d0 GM |
591 | |
592 | Putting so much code in the @code{defvar} form has one disadvantage: | |
593 | it puts the documentation string far away from the line which names the | |
594 | variable. Here's a safe way to avoid that: | |
595 | ||
596 | @example | |
597 | (defvar my-mode-map nil | |
598 | @var{docstring}) | |
599 | (unless my-mode-map | |
600 | (let ((map (make-sparse-keymap))) | |
601 | (define-key map "\C-c\C-a" 'my-command) | |
602 | @dots{} | |
603 | (setq my-mode-map map))) | |
604 | @end example | |
605 | ||
606 | @noindent | |
607 | This has all the same advantages as putting the initialization inside | |
608 | the @code{defvar}, except that you must type @kbd{C-M-x} twice, once on | |
609 | each form, if you do want to reinitialize the variable. | |
610 | ||
b8d4c8d0 GM |
611 | @node Accessing Variables |
612 | @section Accessing Variable Values | |
613 | ||
614 | The usual way to reference a variable is to write the symbol which | |
1021c761 CY |
615 | names it. @xref{Symbol Forms}. |
616 | ||
617 | Occasionally, you may want to reference a variable which is only | |
618 | determined at run time. In that case, you cannot specify the variable | |
619 | name in the text of the program. You can use the @code{symbol-value} | |
620 | function to extract the value. | |
b8d4c8d0 GM |
621 | |
622 | @defun symbol-value symbol | |
735cc5ca CY |
623 | This function returns the value stored in @var{symbol}'s value cell. |
624 | This is where the variable's current (dynamic) value is stored. If | |
625 | the variable has no local binding, this is simply its global value. | |
626 | If the variable is void, a @code{void-variable} error is signaled. | |
1021c761 CY |
627 | |
628 | If the variable is lexically bound, the value reported by | |
735cc5ca CY |
629 | @code{symbol-value} is not necessarily the same as the variable's |
630 | lexical value, which is determined by the lexical environment rather | |
631 | than the symbol's value cell. @xref{Variable Scoping}. | |
b8d4c8d0 GM |
632 | |
633 | @example | |
634 | @group | |
635 | (setq abracadabra 5) | |
636 | @result{} 5 | |
637 | @end group | |
638 | @group | |
639 | (setq foo 9) | |
640 | @result{} 9 | |
641 | @end group | |
642 | ||
643 | @group | |
644 | ;; @r{Here the symbol @code{abracadabra}} | |
645 | ;; @r{is the symbol whose value is examined.} | |
646 | (let ((abracadabra 'foo)) | |
647 | (symbol-value 'abracadabra)) | |
648 | @result{} foo | |
649 | @end group | |
650 | ||
651 | @group | |
652 | ;; @r{Here, the value of @code{abracadabra},} | |
653 | ;; @r{which is @code{foo},} | |
654 | ;; @r{is the symbol whose value is examined.} | |
655 | (let ((abracadabra 'foo)) | |
656 | (symbol-value abracadabra)) | |
657 | @result{} 9 | |
658 | @end group | |
659 | ||
660 | @group | |
661 | (symbol-value 'abracadabra) | |
662 | @result{} 5 | |
663 | @end group | |
664 | @end example | |
b8d4c8d0 GM |
665 | @end defun |
666 | ||
667 | @node Setting Variables | |
1021c761 | 668 | @section Setting Variable Values |
b8d4c8d0 GM |
669 | |
670 | The usual way to change the value of a variable is with the special | |
671 | form @code{setq}. When you need to compute the choice of variable at | |
672 | run time, use the function @code{set}. | |
673 | ||
674 | @defspec setq [symbol form]@dots{} | |
675 | This special form is the most common method of changing a variable's | |
676 | value. Each @var{symbol} is given a new value, which is the result of | |
1021c761 CY |
677 | evaluating the corresponding @var{form}. The current binding of the |
678 | symbol is changed. | |
b8d4c8d0 GM |
679 | |
680 | @code{setq} does not evaluate @var{symbol}; it sets the symbol that you | |
681 | write. We say that this argument is @dfn{automatically quoted}. The | |
16152b76 | 682 | @samp{q} in @code{setq} stands for ``quoted''. |
b8d4c8d0 GM |
683 | |
684 | The value of the @code{setq} form is the value of the last @var{form}. | |
685 | ||
686 | @example | |
687 | @group | |
688 | (setq x (1+ 2)) | |
689 | @result{} 3 | |
690 | @end group | |
691 | x ; @r{@code{x} now has a global value.} | |
692 | @result{} 3 | |
693 | @group | |
694 | (let ((x 5)) | |
695 | (setq x 6) ; @r{The local binding of @code{x} is set.} | |
696 | x) | |
697 | @result{} 6 | |
698 | @end group | |
699 | x ; @r{The global value is unchanged.} | |
700 | @result{} 3 | |
701 | @end example | |
702 | ||
703 | Note that the first @var{form} is evaluated, then the first | |
704 | @var{symbol} is set, then the second @var{form} is evaluated, then the | |
705 | second @var{symbol} is set, and so on: | |
706 | ||
707 | @example | |
708 | @group | |
709 | (setq x 10 ; @r{Notice that @code{x} is set before} | |
710 | y (1+ x)) ; @r{the value of @code{y} is computed.} | |
711 | @result{} 11 | |
712 | @end group | |
713 | @end example | |
714 | @end defspec | |
715 | ||
716 | @defun set symbol value | |
1021c761 CY |
717 | This function puts @var{value} in the value cell of @var{symbol}. |
718 | Since it is a function rather than a special form, the expression | |
719 | written for @var{symbol} is evaluated to obtain the symbol to set. | |
720 | The return value is @var{value}. | |
721 | ||
722 | When dynamic variable binding is in effect (the default), @code{set} | |
723 | has the same effect as @code{setq}, apart from the fact that | |
724 | @code{set} evaluates its @var{symbol} argument whereas @code{setq} | |
725 | does not. But when a variable is lexically bound, @code{set} affects | |
726 | its @emph{dynamic} value, whereas @code{setq} affects its current | |
727 | (lexical) value. @xref{Variable Scoping}. | |
b8d4c8d0 GM |
728 | |
729 | @example | |
730 | @group | |
731 | (set one 1) | |
732 | @error{} Symbol's value as variable is void: one | |
733 | @end group | |
734 | @group | |
735 | (set 'one 1) | |
736 | @result{} 1 | |
737 | @end group | |
738 | @group | |
739 | (set 'two 'one) | |
740 | @result{} one | |
741 | @end group | |
742 | @group | |
743 | (set two 2) ; @r{@code{two} evaluates to symbol @code{one}.} | |
744 | @result{} 2 | |
745 | @end group | |
746 | @group | |
747 | one ; @r{So it is @code{one} that was set.} | |
748 | @result{} 2 | |
749 | (let ((one 1)) ; @r{This binding of @code{one} is set,} | |
750 | (set 'one 3) ; @r{not the global value.} | |
751 | one) | |
752 | @result{} 3 | |
753 | @end group | |
754 | @group | |
755 | one | |
756 | @result{} 2 | |
757 | @end group | |
758 | @end example | |
759 | ||
760 | If @var{symbol} is not actually a symbol, a @code{wrong-type-argument} | |
761 | error is signaled. | |
762 | ||
763 | @example | |
764 | (set '(x y) 'z) | |
765 | @error{} Wrong type argument: symbolp, (x y) | |
766 | @end example | |
b8d4c8d0 GM |
767 | @end defun |
768 | ||
769 | @node Variable Scoping | |
770 | @section Scoping Rules for Variable Bindings | |
362397ed | 771 | @cindex scoping rule |
b8d4c8d0 | 772 | |
1021c761 CY |
773 | When you create a local binding for a variable, that binding takes |
774 | effect only within a limited portion of the program (@pxref{Local | |
775 | Variables}). This section describes exactly what this means. | |
b8d4c8d0 GM |
776 | |
777 | @cindex scope | |
778 | @cindex extent | |
1021c761 CY |
779 | Each local binding has a certain @dfn{scope} and @dfn{extent}. |
780 | @dfn{Scope} refers to @emph{where} in the textual source code the | |
781 | binding can be accessed. @dfn{Extent} refers to @emph{when}, as the | |
782 | program is executing, the binding exists. | |
783 | ||
784 | @cindex dynamic binding | |
362397ed | 785 | @cindex dynamic scope |
1021c761 CY |
786 | @cindex dynamic extent |
787 | By default, the local bindings that Emacs creates are @dfn{dynamic | |
362397ed CY |
788 | bindings}. Such a binding has @dfn{dynamic scope}, meaning that any |
789 | part of the program can potentially access the variable binding. It | |
790 | also has @dfn{dynamic extent}, meaning that the binding lasts only | |
1021c761 CY |
791 | while the binding construct (such as the body of a @code{let} form) is |
792 | being executed. | |
793 | ||
794 | @cindex lexical binding | |
795 | @cindex lexical scope | |
796 | @cindex indefinite extent | |
797 | Emacs can optionally create @dfn{lexical bindings}. A lexical | |
798 | binding has @dfn{lexical scope}, meaning that any reference to the | |
362397ed CY |
799 | variable must be located textually within the binding |
800 | construct@footnote{With some exceptions; for instance, a lexical | |
801 | binding can also be accessed from the Lisp debugger.}. It also has | |
802 | @dfn{indefinite extent}, meaning that under some circumstances the | |
803 | binding can live on even after the binding construct has finished | |
804 | executing, by means of special objects called @dfn{closures}. | |
1021c761 CY |
805 | |
806 | The following subsections describe dynamic binding and lexical | |
807 | binding in greater detail, and how to enable lexical binding in Emacs | |
808 | Lisp programs. | |
b8d4c8d0 GM |
809 | |
810 | @menu | |
1021c761 CY |
811 | * Dynamic Binding:: The default for binding local variables in Emacs. |
812 | * Dynamic Binding Tips:: Avoiding problems with dynamic binding. | |
813 | * Lexical Binding:: A different type of local variable binding. | |
814 | * Using Lexical Binding:: How to enable lexical binding. | |
b8d4c8d0 GM |
815 | @end menu |
816 | ||
1021c761 CY |
817 | @node Dynamic Binding |
818 | @subsection Dynamic Binding | |
819 | ||
820 | By default, the local variable bindings made by Emacs are dynamic | |
821 | bindings. When a variable is dynamically bound, its current binding | |
822 | at any point in the execution of the Lisp program is simply the most | |
823 | recently-created dynamic local binding for that symbol, or the global | |
824 | binding if there is no such local binding. | |
b8d4c8d0 | 825 | |
362397ed CY |
826 | Dynamic bindings have dynamic scope and extent, as shown by the |
827 | following example: | |
b8d4c8d0 GM |
828 | |
829 | @example | |
830 | @group | |
1021c761 | 831 | (defvar x -99) ; @r{@code{x} receives an initial value of -99.} |
b8d4c8d0 | 832 | |
1021c761 CY |
833 | (defun getx () |
834 | x) ; @r{@code{x} is used ``free'' in this function.} | |
835 | ||
836 | (let ((x 1)) ; @r{@code{x} is dynamically bound.} | |
837 | (getx)) | |
838 | @result{} 1 | |
839 | ||
840 | ;; @r{After the @code{let} form finishes, @code{x} reverts to its} | |
841 | ;; @r{previous value, which is -99.} | |
842 | ||
843 | (getx) | |
844 | @result{} -99 | |
b8d4c8d0 GM |
845 | @end group |
846 | @end example | |
847 | ||
1021c761 CY |
848 | @noindent |
849 | The function @code{getx} refers to @code{x}. This is a ``free'' | |
850 | reference, in the sense that there is no binding for @code{x} within | |
851 | that @code{defun} construct itself. When we call @code{getx} from | |
852 | within a @code{let} form in which @code{x} is (dynamically) bound, it | |
362397ed CY |
853 | retrieves the local value (i.e., 1). But when we call @code{getx} |
854 | outside the @code{let} form, it retrieves the global value (i.e., | |
855 | -99). | |
b8d4c8d0 | 856 | |
1021c761 CY |
857 | Here is another example, which illustrates setting a dynamically |
858 | bound variable using @code{setq}: | |
b8d4c8d0 GM |
859 | |
860 | @example | |
861 | @group | |
1021c761 CY |
862 | (defvar x -99) ; @r{@code{x} receives an initial value of -99.} |
863 | ||
864 | (defun addx () | |
865 | (setq x (1+ x))) ; @r{Add 1 to @code{x} and return its new value.} | |
866 | ||
867 | (let ((x 1)) | |
868 | (addx) | |
869 | (addx)) | |
870 | @result{} 3 ; @r{The two @code{addx} calls add to @code{x} twice.} | |
871 | ||
872 | ;; @r{After the @code{let} form finishes, @code{x} reverts to its} | |
873 | ;; @r{previous value, which is -99.} | |
874 | ||
875 | (addx) | |
876 | @result{} -98 | |
b8d4c8d0 GM |
877 | @end group |
878 | @end example | |
879 | ||
1021c761 CY |
880 | Dynamic binding is implemented in Emacs Lisp in a simple way. Each |
881 | symbol has a value cell, which specifies its current dynamic value (or | |
882 | absence of value). @xref{Symbol Components}. When a symbol is given | |
883 | a dynamic local binding, Emacs records the contents of the value cell | |
884 | (or absence thereof) in a stack, and stores the new local value in the | |
885 | value cell. When the binding construct finishes executing, Emacs pops | |
886 | the old value off the stack, and puts it in the value cell. | |
b8d4c8d0 | 887 | |
1021c761 CY |
888 | @node Dynamic Binding Tips |
889 | @subsection Proper Use of Dynamic Binding | |
b8d4c8d0 | 890 | |
1021c761 CY |
891 | Dynamic binding is a powerful feature, as it allows programs to |
892 | refer to variables that are not defined within their local textual | |
893 | scope. However, if used without restraint, this can also make | |
894 | programs hard to understand. There are two clean ways to use this | |
895 | technique: | |
b8d4c8d0 | 896 | |
1021c761 CY |
897 | @itemize @bullet |
898 | @item | |
899 | If a variable has no global definition, use it as a local variable | |
362397ed CY |
900 | only within a binding construct, such as the body of the @code{let} |
901 | form where the variable was bound. If this convention is followed | |
902 | consistently throughout a program, the value of the variable will not | |
903 | affect, nor be affected by, any uses of the same variable symbol | |
904 | elsewhere in the program. | |
1021c761 CY |
905 | |
906 | @item | |
907 | Otherwise, define the variable with @code{defvar}, @code{defconst}, or | |
908 | @code{defcustom}. @xref{Defining Variables}. Usually, the definition | |
909 | should be at top-level in an Emacs Lisp file. As far as possible, it | |
910 | should include a documentation string which explains the meaning and | |
911 | purpose of the variable. You should also choose the variable's name | |
912 | to avoid name conflicts (@pxref{Coding Conventions}). | |
913 | ||
914 | Then you can bind the variable anywhere in a program, knowing reliably | |
915 | what the effect will be. Wherever you encounter the variable, it will | |
1df7defd | 916 | be easy to refer back to the definition, e.g., via the @kbd{C-h v} |
1021c761 CY |
917 | command (provided the variable definition has been loaded into Emacs). |
918 | @xref{Name Help,,, emacs, The GNU Emacs Manual}. | |
919 | ||
920 | For example, it is common to use local bindings for customizable | |
921 | variables like @code{case-fold-search}: | |
b8d4c8d0 GM |
922 | |
923 | @example | |
1021c761 CY |
924 | @group |
925 | (defun search-for-abc () | |
926 | "Search for the string \"abc\", ignoring case differences." | |
927 | (let ((case-fold-search nil)) | |
928 | (re-search-forward "abc"))) | |
929 | @end group | |
b8d4c8d0 | 930 | @end example |
1021c761 | 931 | @end itemize |
b8d4c8d0 | 932 | |
1021c761 CY |
933 | @node Lexical Binding |
934 | @subsection Lexical Binding | |
b8d4c8d0 | 935 | |
362397ed CY |
936 | Lexical binding was introduced to Emacs, as an optional feature, in |
937 | version 24.1. We expect its importance to increase in the future. | |
938 | Lexical binding opens up many more opportunities for optimization, so | |
939 | programs using it are likely to run faster in future Emacs versions. | |
940 | Lexical binding is also more compatible with concurrency, which we | |
941 | want to add to Emacs in the future. | |
b8d4c8d0 | 942 | |
362397ed CY |
943 | A lexically-bound variable has @dfn{lexical scope}, meaning that any |
944 | reference to the variable must be located textually within the binding | |
945 | construct. Here is an example | |
1021c761 CY |
946 | @iftex |
947 | (see the next subsection, for how to actually enable lexical binding): | |
948 | @end iftex | |
949 | @ifnottex | |
950 | (@pxref{Using Lexical Binding}, for how to actually enable lexical binding): | |
951 | @end ifnottex | |
b8d4c8d0 | 952 | |
1021c761 CY |
953 | @example |
954 | @group | |
1021c761 CY |
955 | (let ((x 1)) ; @r{@code{x} is lexically bound.} |
956 | (+ x 3)) | |
957 | @result{} 4 | |
b8d4c8d0 | 958 | |
735cc5ca CY |
959 | (defun getx () |
960 | x) ; @r{@code{x} is used ``free'' in this function.} | |
961 | ||
1021c761 CY |
962 | (let ((x 1)) ; @r{@code{x} is lexically bound.} |
963 | (getx)) | |
964 | @error{} Symbol's value as variable is void: x | |
965 | @end group | |
966 | @end example | |
b8d4c8d0 | 967 | |
1021c761 CY |
968 | @noindent |
969 | Here, the variable @code{x} has no global value. When it is lexically | |
970 | bound within a @code{let} form, it can be used in the textual confines | |
971 | of that @code{let} form. But it can @emph{not} be used from within a | |
972 | @code{getx} function called from the @code{let} form, since the | |
973 | function definition of @code{getx} occurs outside the @code{let} form | |
974 | itself. | |
975 | ||
976 | @cindex lexical environment | |
977 | Here is how lexical binding works. Each binding construct defines a | |
978 | @dfn{lexical environment}, specifying the symbols that are bound | |
979 | within the construct and their local values. When the Lisp evaluator | |
980 | wants the current value of a variable, it looks first in the lexical | |
981 | environment; if the variable is not specified in there, it looks in | |
735cc5ca | 982 | the symbol's value cell, where the dynamic value is stored. |
1021c761 | 983 | |
362397ed CY |
984 | (Internally, the lexical environment is an alist of symbol-value |
985 | pairs, with the final element in the alist being the symbol @code{t} | |
986 | rather than a cons cell. Such an alist can be passed as the second | |
987 | argument to the @code{eval} function, in order to specify a lexical | |
988 | environment in which to evaluate a form. @xref{Eval}. Most Emacs | |
989 | Lisp programs, however, should not interact directly with lexical | |
990 | environments in this way; only specialized programs like debuggers.) | |
991 | ||
a08eadfe | 992 | @cindex closures, example of using |
1021c761 CY |
993 | Lexical bindings have indefinite extent. Even after a binding |
994 | construct has finished executing, its lexical environment can be | |
995 | ``kept around'' in Lisp objects called @dfn{closures}. A closure is | |
a08eadfe | 996 | created when you define a named or anonymous function with lexical |
735cc5ca | 997 | binding enabled. @xref{Closures}, for details. |
1021c761 | 998 | |
735cc5ca CY |
999 | When a closure is called as a function, any lexical variable |
1000 | references within its definition use the retained lexical environment. | |
1001 | Here is an example: | |
b8d4c8d0 | 1002 | |
1021c761 CY |
1003 | @example |
1004 | (defvar my-ticker nil) ; @r{We will use this dynamically bound} | |
1005 | ; @r{variable to store a closure.} | |
b8d4c8d0 | 1006 | |
1021c761 CY |
1007 | (let ((x 0)) ; @r{@code{x} is lexically bound.} |
1008 | (setq my-ticker (lambda () | |
1009 | (setq x (1+ x))))) | |
1010 | @result{} (closure ((x . 0) t) () | |
611e8a48 | 1011 | (setq x (1+ x))) |
b8d4c8d0 | 1012 | |
1021c761 CY |
1013 | (funcall my-ticker) |
1014 | @result{} 1 | |
b8d4c8d0 | 1015 | |
1021c761 CY |
1016 | (funcall my-ticker) |
1017 | @result{} 2 | |
b8d4c8d0 | 1018 | |
1021c761 CY |
1019 | (funcall my-ticker) |
1020 | @result{} 3 | |
d032d5e7 | 1021 | |
1021c761 CY |
1022 | x ; @r{Note that @code{x} has no global value.} |
1023 | @error{} Symbol's value as variable is void: x | |
1024 | @end example | |
d032d5e7 | 1025 | |
1021c761 CY |
1026 | @noindent |
1027 | The @code{let} binding defines a lexical environment in which the | |
1028 | variable @code{x} is locally bound to 0. Within this binding | |
1029 | construct, we define a lambda expression which increments @code{x} by | |
1030 | one and returns the incremented value. This lambda expression is | |
1031 | automatically turned into a closure, in which the lexical environment | |
1032 | lives on even after the @code{let} binding construct has exited. Each | |
1033 | time we evaluate the closure, it increments @code{x}, using the | |
1034 | binding of @code{x} in that lexical environment. | |
1035 | ||
1036 | Note that functions like @code{symbol-value}, @code{boundp}, and | |
1037 | @code{set} only retrieve or modify a variable's dynamic binding | |
1df7defd | 1038 | (i.e., the contents of its symbol's value cell). Also, the code in |
1021c761 CY |
1039 | the body of a @code{defun} or @code{defmacro} cannot refer to |
1040 | surrounding lexical variables. | |
1041 | ||
1021c761 CY |
1042 | @node Using Lexical Binding |
1043 | @subsection Using Lexical Binding | |
1044 | ||
1045 | When loading an Emacs Lisp file or evaluating a Lisp buffer, lexical | |
1046 | binding is enabled if the buffer-local variable @code{lexical-binding} | |
1047 | is non-@code{nil}: | |
d032d5e7 SM |
1048 | |
1049 | @defvar lexical-binding | |
1021c761 CY |
1050 | If this buffer-local variable is non-@code{nil}, Emacs Lisp files and |
1051 | buffers are evaluated using lexical binding instead of dynamic | |
1052 | binding. (However, special variables are still dynamically bound; see | |
1053 | below.) If @code{nil}, dynamic binding is used for all local | |
1054 | variables. This variable is typically set for a whole Emacs Lisp | |
1055 | file, as a file local variable (@pxref{File Local Variables}). | |
48da7392 GM |
1056 | Note that unlike other such variables, this one must be set in the |
1057 | first line of a file. | |
d032d5e7 SM |
1058 | @end defvar |
1059 | ||
1021c761 CY |
1060 | @noindent |
1061 | When evaluating Emacs Lisp code directly using an @code{eval} call, | |
1062 | lexical binding is enabled if the @var{lexical} argument to | |
1063 | @code{eval} is non-@code{nil}. @xref{Eval}. | |
1064 | ||
1065 | @cindex special variables | |
1066 | Even when lexical binding is enabled, certain variables will | |
1067 | continue to be dynamically bound. These are called @dfn{special | |
1068 | variables}. Every variable that has been defined with @code{defvar}, | |
1069 | @code{defcustom} or @code{defconst} is a special variable | |
1070 | (@pxref{Defining Variables}). All other variables are subject to | |
1071 | lexical binding. | |
1072 | ||
151d9088 | 1073 | @defun special-variable-p symbol |
1021c761 | 1074 | This function returns non-@code{nil} if @var{symbol} is a special |
1df7defd | 1075 | variable (i.e., it has a @code{defvar}, @code{defcustom}, or |
1021c761 CY |
1076 | @code{defconst} variable definition). Otherwise, the return value is |
1077 | @code{nil}. | |
d032d5e7 SM |
1078 | @end defun |
1079 | ||
1021c761 CY |
1080 | The use of a special variable as a formal argument in a function is |
1081 | discouraged. Doing so gives rise to unspecified behavior when lexical | |
1082 | binding mode is enabled (it may use lexical binding sometimes, and | |
1083 | dynamic binding other times). | |
1084 | ||
362397ed CY |
1085 | Converting an Emacs Lisp program to lexical binding is easy. First, |
1086 | add a file-local variable setting of @code{lexical-binding} to | |
1087 | @code{t} in the header line of the Emacs Lisp source file (@pxref{File | |
1088 | Local Variables}). Second, check that every variable in the program | |
1089 | which needs to be dynamically bound has a variable definition, so that | |
1090 | it is not inadvertently bound lexically. | |
1021c761 | 1091 | |
362397ed CY |
1092 | @cindex free variable |
1093 | @cindex unused lexical variable | |
1021c761 CY |
1094 | A simple way to find out which variables need a variable definition |
1095 | is to byte-compile the source file. @xref{Byte Compilation}. If a | |
1096 | non-special variable is used outside of a @code{let} form, the | |
1097 | byte-compiler will warn about reference or assignment to a ``free | |
1098 | variable''. If a non-special variable is bound but not used within a | |
1099 | @code{let} form, the byte-compiler will warn about an ``unused lexical | |
1100 | variable''. The byte-compiler will also issue a warning if you use a | |
1101 | special variable as a function argument. | |
1102 | ||
1103 | (To silence byte-compiler warnings about unused variables, just use | |
1104 | a variable name that start with an underscore. The byte-compiler | |
1105 | interprets this as an indication that this is a variable known not to | |
1106 | be used.) | |
d032d5e7 | 1107 | |
b8d4c8d0 GM |
1108 | @node Buffer-Local Variables |
1109 | @section Buffer-Local Variables | |
1110 | @cindex variable, buffer-local | |
1111 | @cindex buffer-local variables | |
1112 | ||
1113 | Global and local variable bindings are found in most programming | |
e388c68f RS |
1114 | languages in one form or another. Emacs, however, also supports |
1115 | additional, unusual kinds of variable binding, such as | |
1116 | @dfn{buffer-local} bindings, which apply only in one buffer. Having | |
1117 | different values for a variable in different buffers is an important | |
32770114 | 1118 | customization method. (Variables can also have bindings that are |
c830e5ae | 1119 | local to each terminal. @xref{Multiple Terminals}.) |
b8d4c8d0 GM |
1120 | |
1121 | @menu | |
d032d5e7 SM |
1122 | * Intro to Buffer-Local:: Introduction and concepts. |
1123 | * Creating Buffer-Local:: Creating and destroying buffer-local bindings. | |
1124 | * Default Value:: The default value is seen in buffers | |
b8d4c8d0 GM |
1125 | that don't have their own buffer-local values. |
1126 | @end menu | |
1127 | ||
1128 | @node Intro to Buffer-Local | |
1129 | @subsection Introduction to Buffer-Local Variables | |
1130 | ||
1131 | A buffer-local variable has a buffer-local binding associated with a | |
1132 | particular buffer. The binding is in effect when that buffer is | |
1133 | current; otherwise, it is not in effect. If you set the variable while | |
1134 | a buffer-local binding is in effect, the new value goes in that binding, | |
1135 | so its other bindings are unchanged. This means that the change is | |
1136 | visible only in the buffer where you made it. | |
1137 | ||
1138 | The variable's ordinary binding, which is not associated with any | |
1139 | specific buffer, is called the @dfn{default binding}. In most cases, | |
1140 | this is the global binding. | |
1141 | ||
1142 | A variable can have buffer-local bindings in some buffers but not in | |
1143 | other buffers. The default binding is shared by all the buffers that | |
1144 | don't have their own bindings for the variable. (This includes all | |
1145 | newly-created buffers.) If you set the variable in a buffer that does | |
e388c68f | 1146 | not have a buffer-local binding for it, this sets the default binding, |
b8d4c8d0 GM |
1147 | so the new value is visible in all the buffers that see the default |
1148 | binding. | |
1149 | ||
1150 | The most common use of buffer-local bindings is for major modes to change | |
1151 | variables that control the behavior of commands. For example, C mode and | |
1152 | Lisp mode both set the variable @code{paragraph-start} to specify that only | |
1153 | blank lines separate paragraphs. They do this by making the variable | |
1154 | buffer-local in the buffer that is being put into C mode or Lisp mode, and | |
1155 | then setting it to the new value for that mode. @xref{Major Modes}. | |
1156 | ||
1157 | The usual way to make a buffer-local binding is with | |
1158 | @code{make-local-variable}, which is what major mode commands typically | |
1159 | use. This affects just the current buffer; all other buffers (including | |
1160 | those yet to be created) will continue to share the default value unless | |
1161 | they are explicitly given their own buffer-local bindings. | |
1162 | ||
1163 | @cindex automatically buffer-local | |
1164 | A more powerful operation is to mark the variable as | |
1165 | @dfn{automatically buffer-local} by calling | |
1166 | @code{make-variable-buffer-local}. You can think of this as making the | |
1167 | variable local in all buffers, even those yet to be created. More | |
1168 | precisely, the effect is that setting the variable automatically makes | |
1169 | the variable local to the current buffer if it is not already so. All | |
1170 | buffers start out by sharing the default value of the variable as usual, | |
1171 | but setting the variable creates a buffer-local binding for the current | |
1172 | buffer. The new value is stored in the buffer-local binding, leaving | |
1173 | the default binding untouched. This means that the default value cannot | |
1174 | be changed with @code{setq} in any buffer; the only way to change it is | |
1175 | with @code{setq-default}. | |
1176 | ||
e388c68f | 1177 | @strong{Warning:} When a variable has buffer-local |
b8d4c8d0 GM |
1178 | bindings in one or more buffers, @code{let} rebinds the binding that's |
1179 | currently in effect. For instance, if the current buffer has a | |
1180 | buffer-local value, @code{let} temporarily rebinds that. If no | |
e388c68f | 1181 | buffer-local bindings are in effect, @code{let} rebinds |
b8d4c8d0 GM |
1182 | the default value. If inside the @code{let} you then change to a |
1183 | different current buffer in which a different binding is in effect, | |
1184 | you won't see the @code{let} binding any more. And if you exit the | |
1185 | @code{let} while still in the other buffer, you won't see the | |
1186 | unbinding occur (though it will occur properly). Here is an example | |
1187 | to illustrate: | |
1188 | ||
1189 | @example | |
1190 | @group | |
1191 | (setq foo 'g) | |
1192 | (set-buffer "a") | |
1193 | (make-local-variable 'foo) | |
1194 | @end group | |
1195 | (setq foo 'a) | |
1196 | (let ((foo 'temp)) | |
1197 | ;; foo @result{} 'temp ; @r{let binding in buffer @samp{a}} | |
1198 | (set-buffer "b") | |
1199 | ;; foo @result{} 'g ; @r{the global value since foo is not local in @samp{b}} | |
1200 | @var{body}@dots{}) | |
1201 | @group | |
1202 | foo @result{} 'g ; @r{exiting restored the local value in buffer @samp{a},} | |
1203 | ; @r{but we don't see that in buffer @samp{b}} | |
1204 | @end group | |
1205 | @group | |
1206 | (set-buffer "a") ; @r{verify the local value was restored} | |
1207 | foo @result{} 'a | |
1208 | @end group | |
1209 | @end example | |
1210 | ||
735cc5ca CY |
1211 | @noindent |
1212 | Note that references to @code{foo} in @var{body} access the | |
b8d4c8d0 GM |
1213 | buffer-local binding of buffer @samp{b}. |
1214 | ||
1215 | When a file specifies local variable values, these become buffer-local | |
1216 | values when you visit the file. @xref{File Variables,,, emacs, The | |
1217 | GNU Emacs Manual}. | |
1218 | ||
c830e5ae CY |
1219 | A buffer-local variable cannot be made terminal-local |
1220 | (@pxref{Multiple Terminals}). | |
0f7766a4 | 1221 | |
b8d4c8d0 GM |
1222 | @node Creating Buffer-Local |
1223 | @subsection Creating and Deleting Buffer-Local Bindings | |
1224 | ||
1225 | @deffn Command make-local-variable variable | |
1226 | This function creates a buffer-local binding in the current buffer for | |
1227 | @var{variable} (a symbol). Other buffers are not affected. The value | |
1228 | returned is @var{variable}. | |
1229 | ||
b8d4c8d0 GM |
1230 | The buffer-local value of @var{variable} starts out as the same value |
1231 | @var{variable} previously had. If @var{variable} was void, it remains | |
1232 | void. | |
1233 | ||
1234 | @example | |
1235 | @group | |
1236 | ;; @r{In buffer @samp{b1}:} | |
1237 | (setq foo 5) ; @r{Affects all buffers.} | |
1238 | @result{} 5 | |
1239 | @end group | |
1240 | @group | |
1241 | (make-local-variable 'foo) ; @r{Now it is local in @samp{b1}.} | |
1242 | @result{} foo | |
1243 | @end group | |
1244 | @group | |
1245 | foo ; @r{That did not change} | |
1246 | @result{} 5 ; @r{the value.} | |
1247 | @end group | |
1248 | @group | |
1249 | (setq foo 6) ; @r{Change the value} | |
1250 | @result{} 6 ; @r{in @samp{b1}.} | |
1251 | @end group | |
1252 | @group | |
1253 | foo | |
1254 | @result{} 6 | |
1255 | @end group | |
1256 | ||
1257 | @group | |
1258 | ;; @r{In buffer @samp{b2}, the value hasn't changed.} | |
c57008f6 | 1259 | (with-current-buffer "b2" |
b8d4c8d0 GM |
1260 | foo) |
1261 | @result{} 5 | |
1262 | @end group | |
1263 | @end example | |
1264 | ||
1265 | Making a variable buffer-local within a @code{let}-binding for that | |
1266 | variable does not work reliably, unless the buffer in which you do this | |
1267 | is not current either on entry to or exit from the @code{let}. This is | |
1268 | because @code{let} does not distinguish between different kinds of | |
1269 | bindings; it knows only which variable the binding was made for. | |
1270 | ||
c830e5ae CY |
1271 | If the variable is terminal-local (@pxref{Multiple Terminals}), this |
1272 | function signals an error. Such variables cannot have buffer-local | |
1273 | bindings as well. | |
b8d4c8d0 GM |
1274 | |
1275 | @strong{Warning:} do not use @code{make-local-variable} for a hook | |
1276 | variable. The hook variables are automatically made buffer-local as | |
1277 | needed if you use the @var{local} argument to @code{add-hook} or | |
1278 | @code{remove-hook}. | |
1279 | @end deffn | |
1280 | ||
7c08f8ba CY |
1281 | @defmac setq-local variable value |
1282 | This macro creates a buffer-local binding in the current buffer for | |
1283 | @var{variable}, and gives it the buffer-local value @var{value}. It | |
1284 | is equivalent to calling @code{make-local-variable} followed by | |
1285 | @code{setq}. @var{variable} should be an unquoted symbol. | |
1286 | @end defmac | |
1287 | ||
b8d4c8d0 GM |
1288 | @deffn Command make-variable-buffer-local variable |
1289 | This function marks @var{variable} (a symbol) automatically | |
1290 | buffer-local, so that any subsequent attempt to set it will make it | |
6a43ef8e CY |
1291 | local to the current buffer at the time. Unlike |
1292 | @code{make-local-variable}, with which it is often confused, this | |
1293 | cannot be undone, and affects the behavior of the variable in all | |
1294 | buffers. | |
b8d4c8d0 GM |
1295 | |
1296 | A peculiar wrinkle of this feature is that binding the variable (with | |
1297 | @code{let} or other binding constructs) does not create a buffer-local | |
1298 | binding for it. Only setting the variable (with @code{set} or | |
1299 | @code{setq}), while the variable does not have a @code{let}-style | |
1300 | binding that was made in the current buffer, does so. | |
1301 | ||
1302 | If @var{variable} does not have a default value, then calling this | |
1303 | command will give it a default value of @code{nil}. If @var{variable} | |
1304 | already has a default value, that value remains unchanged. | |
1305 | Subsequently calling @code{makunbound} on @var{variable} will result | |
1306 | in a void buffer-local value and leave the default value unaffected. | |
1307 | ||
1308 | The value returned is @var{variable}. | |
1309 | ||
1310 | @strong{Warning:} Don't assume that you should use | |
1311 | @code{make-variable-buffer-local} for user-option variables, simply | |
1312 | because users @emph{might} want to customize them differently in | |
1313 | different buffers. Users can make any variable local, when they wish | |
1314 | to. It is better to leave the choice to them. | |
1315 | ||
1316 | The time to use @code{make-variable-buffer-local} is when it is crucial | |
1317 | that no two buffers ever share the same binding. For example, when a | |
1318 | variable is used for internal purposes in a Lisp program which depends | |
1319 | on having separate values in separate buffers, then using | |
1320 | @code{make-variable-buffer-local} can be the best solution. | |
1321 | @end deffn | |
1322 | ||
7c08f8ba CY |
1323 | @defmac defvar-local variable value &optional docstring |
1324 | This macro defines @var{variable} as a variable with initial value | |
1325 | @var{value} and @var{docstring}, and marks it as automatically | |
1326 | buffer-local. It is equivalent to calling @code{defvar} followed by | |
1327 | @code{make-variable-buffer-local}. @var{variable} should be an | |
1328 | unquoted symbol. | |
1329 | @end defmac | |
1330 | ||
b8d4c8d0 GM |
1331 | @defun local-variable-p variable &optional buffer |
1332 | This returns @code{t} if @var{variable} is buffer-local in buffer | |
1333 | @var{buffer} (which defaults to the current buffer); otherwise, | |
1334 | @code{nil}. | |
1335 | @end defun | |
1336 | ||
1337 | @defun local-variable-if-set-p variable &optional buffer | |
1a5432bc CY |
1338 | This returns @code{t} if @var{variable} either has a buffer-local |
1339 | value in buffer @var{buffer}, or is automatically buffer-local. | |
1340 | Otherwise, it returns @code{nil}. If omitted or @code{nil}, | |
1341 | @var{buffer} defaults to the current buffer. | |
b8d4c8d0 GM |
1342 | @end defun |
1343 | ||
1344 | @defun buffer-local-value variable buffer | |
1345 | This function returns the buffer-local binding of @var{variable} (a | |
1346 | symbol) in buffer @var{buffer}. If @var{variable} does not have a | |
1347 | buffer-local binding in buffer @var{buffer}, it returns the default | |
1348 | value (@pxref{Default Value}) of @var{variable} instead. | |
1349 | @end defun | |
1350 | ||
1351 | @defun buffer-local-variables &optional buffer | |
1352 | This function returns a list describing the buffer-local variables in | |
0992bd9c CY |
1353 | buffer @var{buffer}. (If @var{buffer} is omitted, the current buffer |
1354 | is used.) Normally, each list element has the form | |
1355 | @w{@code{(@var{sym} . @var{val})}}, where @var{sym} is a buffer-local | |
1356 | variable (a symbol) and @var{val} is its buffer-local value. But when | |
1357 | a variable's buffer-local binding in @var{buffer} is void, its list | |
1358 | element is just @var{sym}. | |
b8d4c8d0 GM |
1359 | |
1360 | @example | |
1361 | @group | |
1362 | (make-local-variable 'foobar) | |
1363 | (makunbound 'foobar) | |
1364 | (make-local-variable 'bind-me) | |
1365 | (setq bind-me 69) | |
1366 | @end group | |
1367 | (setq lcl (buffer-local-variables)) | |
1368 | ;; @r{First, built-in variables local in all buffers:} | |
1369 | @result{} ((mark-active . nil) | |
1370 | (buffer-undo-list . nil) | |
1371 | (mode-name . "Fundamental") | |
1372 | @dots{} | |
1373 | @group | |
1374 | ;; @r{Next, non-built-in buffer-local variables.} | |
1375 | ;; @r{This one is buffer-local and void:} | |
1376 | foobar | |
1377 | ;; @r{This one is buffer-local and nonvoid:} | |
1378 | (bind-me . 69)) | |
1379 | @end group | |
1380 | @end example | |
1381 | ||
1382 | Note that storing new values into the @sc{cdr}s of cons cells in this | |
1383 | list does @emph{not} change the buffer-local values of the variables. | |
1384 | @end defun | |
1385 | ||
1386 | @deffn Command kill-local-variable variable | |
1387 | This function deletes the buffer-local binding (if any) for | |
1388 | @var{variable} (a symbol) in the current buffer. As a result, the | |
1389 | default binding of @var{variable} becomes visible in this buffer. This | |
1390 | typically results in a change in the value of @var{variable}, since the | |
1391 | default value is usually different from the buffer-local value just | |
1392 | eliminated. | |
1393 | ||
1394 | If you kill the buffer-local binding of a variable that automatically | |
1395 | becomes buffer-local when set, this makes the default value visible in | |
1396 | the current buffer. However, if you set the variable again, that will | |
1397 | once again create a buffer-local binding for it. | |
1398 | ||
1399 | @code{kill-local-variable} returns @var{variable}. | |
1400 | ||
1401 | This function is a command because it is sometimes useful to kill one | |
1402 | buffer-local variable interactively, just as it is useful to create | |
1403 | buffer-local variables interactively. | |
1404 | @end deffn | |
1405 | ||
1406 | @defun kill-all-local-variables | |
1407 | This function eliminates all the buffer-local variable bindings of the | |
ee666f84 EZ |
1408 | current buffer except for variables marked as ``permanent'' and local |
1409 | hook functions that have a non-@code{nil} @code{permanent-local-hook} | |
1410 | property (@pxref{Setting Hooks}). As a result, the buffer will see | |
1411 | the default values of most variables. | |
b8d4c8d0 GM |
1412 | |
1413 | This function also resets certain other information pertaining to the | |
1414 | buffer: it sets the local keymap to @code{nil}, the syntax table to the | |
1415 | value of @code{(standard-syntax-table)}, the case table to | |
1416 | @code{(standard-case-table)}, and the abbrev table to the value of | |
1417 | @code{fundamental-mode-abbrev-table}. | |
1418 | ||
1419 | The very first thing this function does is run the normal hook | |
1420 | @code{change-major-mode-hook} (see below). | |
1421 | ||
1422 | Every major mode command begins by calling this function, which has the | |
1423 | effect of switching to Fundamental mode and erasing most of the effects | |
1424 | of the previous major mode. To ensure that this does its job, the | |
1425 | variables that major modes set should not be marked permanent. | |
1426 | ||
1427 | @code{kill-all-local-variables} returns @code{nil}. | |
1428 | @end defun | |
1429 | ||
1430 | @defvar change-major-mode-hook | |
1431 | The function @code{kill-all-local-variables} runs this normal hook | |
1432 | before it does anything else. This gives major modes a way to arrange | |
1433 | for something special to be done if the user switches to a different | |
1434 | major mode. It is also useful for buffer-specific minor modes | |
1435 | that should be forgotten if the user changes the major mode. | |
1436 | ||
1437 | For best results, make this variable buffer-local, so that it will | |
1438 | disappear after doing its job and will not interfere with the | |
1439 | subsequent major mode. @xref{Hooks}. | |
1440 | @end defvar | |
1441 | ||
b8d4c8d0 GM |
1442 | @cindex permanent local variable |
1443 | A buffer-local variable is @dfn{permanent} if the variable name (a | |
1444 | symbol) has a @code{permanent-local} property that is non-@code{nil}. | |
86ec878a EZ |
1445 | Such variables are unaffected by @code{kill-all-local-variables}, and |
1446 | their local bindings are therefore not cleared by changing major modes. | |
b8d4c8d0 GM |
1447 | Permanent locals are appropriate for data pertaining to where the file |
1448 | came from or how to save it, rather than with how to edit the contents. | |
1449 | ||
1450 | @node Default Value | |
1451 | @subsection The Default Value of a Buffer-Local Variable | |
1452 | @cindex default value | |
1453 | ||
1454 | The global value of a variable with buffer-local bindings is also | |
1455 | called the @dfn{default} value, because it is the value that is in | |
1456 | effect whenever neither the current buffer nor the selected frame has | |
1457 | its own binding for the variable. | |
1458 | ||
1459 | The functions @code{default-value} and @code{setq-default} access and | |
1460 | change a variable's default value regardless of whether the current | |
1461 | buffer has a buffer-local binding. For example, you could use | |
1462 | @code{setq-default} to change the default setting of | |
1463 | @code{paragraph-start} for most buffers; and this would work even when | |
1464 | you are in a C or Lisp mode buffer that has a buffer-local value for | |
1465 | this variable. | |
1466 | ||
1467 | @c Emacs 19 feature | |
1468 | The special forms @code{defvar} and @code{defconst} also set the | |
1469 | default value (if they set the variable at all), rather than any | |
e388c68f | 1470 | buffer-local value. |
b8d4c8d0 GM |
1471 | |
1472 | @defun default-value symbol | |
1473 | This function returns @var{symbol}'s default value. This is the value | |
1474 | that is seen in buffers and frames that do not have their own values for | |
1475 | this variable. If @var{symbol} is not buffer-local, this is equivalent | |
1476 | to @code{symbol-value} (@pxref{Accessing Variables}). | |
1477 | @end defun | |
1478 | ||
1479 | @c Emacs 19 feature | |
1480 | @defun default-boundp symbol | |
1481 | The function @code{default-boundp} tells you whether @var{symbol}'s | |
1482 | default value is nonvoid. If @code{(default-boundp 'foo)} returns | |
1483 | @code{nil}, then @code{(default-value 'foo)} would get an error. | |
1484 | ||
1485 | @code{default-boundp} is to @code{default-value} as @code{boundp} is to | |
1486 | @code{symbol-value}. | |
1487 | @end defun | |
1488 | ||
1489 | @defspec setq-default [symbol form]@dots{} | |
1490 | This special form gives each @var{symbol} a new default value, which is | |
1491 | the result of evaluating the corresponding @var{form}. It does not | |
1492 | evaluate @var{symbol}, but does evaluate @var{form}. The value of the | |
1493 | @code{setq-default} form is the value of the last @var{form}. | |
1494 | ||
1495 | If a @var{symbol} is not buffer-local for the current buffer, and is not | |
1496 | marked automatically buffer-local, @code{setq-default} has the same | |
1497 | effect as @code{setq}. If @var{symbol} is buffer-local for the current | |
1498 | buffer, then this changes the value that other buffers will see (as long | |
1499 | as they don't have a buffer-local value), but not the value that the | |
1500 | current buffer sees. | |
1501 | ||
1502 | @example | |
1503 | @group | |
1504 | ;; @r{In buffer @samp{foo}:} | |
1505 | (make-local-variable 'buffer-local) | |
1506 | @result{} buffer-local | |
1507 | @end group | |
1508 | @group | |
1509 | (setq buffer-local 'value-in-foo) | |
1510 | @result{} value-in-foo | |
1511 | @end group | |
1512 | @group | |
1513 | (setq-default buffer-local 'new-default) | |
1514 | @result{} new-default | |
1515 | @end group | |
1516 | @group | |
1517 | buffer-local | |
1518 | @result{} value-in-foo | |
1519 | @end group | |
1520 | @group | |
1521 | (default-value 'buffer-local) | |
1522 | @result{} new-default | |
1523 | @end group | |
1524 | ||
1525 | @group | |
1526 | ;; @r{In (the new) buffer @samp{bar}:} | |
1527 | buffer-local | |
1528 | @result{} new-default | |
1529 | @end group | |
1530 | @group | |
1531 | (default-value 'buffer-local) | |
1532 | @result{} new-default | |
1533 | @end group | |
1534 | @group | |
1535 | (setq buffer-local 'another-default) | |
1536 | @result{} another-default | |
1537 | @end group | |
1538 | @group | |
1539 | (default-value 'buffer-local) | |
1540 | @result{} another-default | |
1541 | @end group | |
1542 | ||
1543 | @group | |
1544 | ;; @r{Back in buffer @samp{foo}:} | |
1545 | buffer-local | |
1546 | @result{} value-in-foo | |
1547 | (default-value 'buffer-local) | |
1548 | @result{} another-default | |
1549 | @end group | |
1550 | @end example | |
1551 | @end defspec | |
1552 | ||
1553 | @defun set-default symbol value | |
1554 | This function is like @code{setq-default}, except that @var{symbol} is | |
1555 | an ordinary evaluated argument. | |
1556 | ||
1557 | @example | |
1558 | @group | |
1559 | (set-default (car '(a b c)) 23) | |
1560 | @result{} 23 | |
1561 | @end group | |
1562 | @group | |
1563 | (default-value 'a) | |
1564 | @result{} 23 | |
1565 | @end group | |
1566 | @end example | |
1567 | @end defun | |
1568 | ||
b8d4c8d0 GM |
1569 | @node File Local Variables |
1570 | @section File Local Variables | |
1571 | @cindex file local variables | |
1572 | ||
1573 | A file can specify local variable values; Emacs uses these to create | |
1574 | buffer-local bindings for those variables in the buffer visiting that | |
5765e9e0 | 1575 | file. @xref{File Variables, , Local Variables in Files, emacs, The |
32770114 CY |
1576 | GNU Emacs Manual}, for basic information about file-local variables. |
1577 | This section describes the functions and variables that affect how | |
1578 | file-local variables are processed. | |
1579 | ||
1580 | If a file-local variable could specify an arbitrary function or Lisp | |
1581 | expression that would be called later, visiting a file could take over | |
1582 | your Emacs. Emacs protects against this by automatically setting only | |
1583 | those file-local variables whose specified values are known to be | |
1584 | safe. Other file-local variables are set only if the user agrees. | |
b8d4c8d0 | 1585 | |
dd449674 CY |
1586 | For additional safety, @code{read-circle} is temporarily bound to |
1587 | @code{nil} when Emacs reads file-local variables (@pxref{Input | |
1588 | Functions}). This prevents the Lisp reader from recognizing circular | |
1589 | and shared Lisp structures (@pxref{Circular Objects}). | |
1590 | ||
b8d4c8d0 | 1591 | @defopt enable-local-variables |
32770114 | 1592 | This variable controls whether to process file-local variables. |
b8d4c8d0 GM |
1593 | The possible values are: |
1594 | ||
1595 | @table @asis | |
1596 | @item @code{t} (the default) | |
1597 | Set the safe variables, and query (once) about any unsafe variables. | |
1598 | @item @code{:safe} | |
1599 | Set only the safe variables and do not query. | |
1600 | @item @code{:all} | |
1601 | Set all the variables and do not query. | |
1602 | @item @code{nil} | |
1603 | Don't set any variables. | |
1604 | @item anything else | |
1605 | Query (once) about all the variables. | |
1606 | @end table | |
1607 | @end defopt | |
1608 | ||
61086eb6 GM |
1609 | @defvar inhibit-local-variables-regexps |
1610 | This is a list of regular expressions. If a file has a name | |
1611 | matching an element of this list, then it is not scanned for | |
1612 | any form of file-local variable. For examples of why you might want | |
1613 | to use this, @pxref{Auto Major Mode}. | |
1614 | @end defvar | |
1615 | ||
b8d4c8d0 GM |
1616 | @defun hack-local-variables &optional mode-only |
1617 | This function parses, and binds or evaluates as appropriate, any local | |
1618 | variables specified by the contents of the current buffer. The variable | |
1619 | @code{enable-local-variables} has its effect here. However, this | |
1620 | function does not look for the @samp{mode:} local variable in the | |
1621 | @w{@samp{-*-}} line. @code{set-auto-mode} does that, also taking | |
1622 | @code{enable-local-variables} into account (@pxref{Auto Major Mode}). | |
1623 | ||
291703b5 | 1624 | This function works by walking the alist stored in |
3a57591a | 1625 | @code{file-local-variables-alist} and applying each local variable in |
291703b5 EZ |
1626 | turn. It calls @code{before-hack-local-variables-hook} and |
1627 | @code{hack-local-variables-hook} before and after applying the | |
2d3ba9e7 GM |
1628 | variables, respectively. It only calls the before-hook if the alist |
1629 | is non-@code{nil}; it always calls the other hook. This | |
1630 | function ignores a @samp{mode} element if it specifies the same major | |
1631 | mode as the buffer already has. | |
291703b5 | 1632 | |
b8d4c8d0 | 1633 | If the optional argument @var{mode-only} is non-@code{nil}, then all |
eafed945 GM |
1634 | this function does is return a symbol specifying the major mode, |
1635 | if the @w{@samp{-*-}} line or the local variables list specifies one, | |
1636 | and @code{nil} otherwise. It does not set the mode nor any other | |
1637 | file-local variable. | |
b8d4c8d0 GM |
1638 | @end defun |
1639 | ||
291703b5 EZ |
1640 | @defvar file-local-variables-alist |
1641 | This buffer-local variable holds the alist of file-local variable | |
1642 | settings. Each element of the alist is of the form | |
1643 | @w{@code{(@var{var} . @var{value})}}, where @var{var} is a symbol of | |
1644 | the local variable and @var{value} is its value. When Emacs visits a | |
1645 | file, it first collects all the file-local variables into this alist, | |
1646 | and then the @code{hack-local-variables} function applies them one by | |
1647 | one. | |
1648 | @end defvar | |
1649 | ||
1650 | @defvar before-hack-local-variables-hook | |
1651 | Emacs calls this hook immediately before applying file-local variables | |
1652 | stored in @code{file-local-variables-alist}. | |
1653 | @end defvar | |
1654 | ||
1655 | @defvar hack-local-variables-hook | |
1656 | Emacs calls this hook immediately after it finishes applying | |
1657 | file-local variables stored in @code{file-local-variables-alist}. | |
1658 | @end defvar | |
1659 | ||
b8d4c8d0 GM |
1660 | @cindex safe local variable |
1661 | You can specify safe values for a variable with a | |
32770114 CY |
1662 | @code{safe-local-variable} property. The property has to be a |
1663 | function of one argument; any value is safe if the function returns | |
1664 | non-@code{nil} given that value. Many commonly-encountered file | |
1665 | variables have @code{safe-local-variable} properties; these include | |
1666 | @code{fill-column}, @code{fill-prefix}, and @code{indent-tabs-mode}. | |
1667 | For boolean-valued variables that are safe, use @code{booleanp} as the | |
a8544941 | 1668 | property value. |
b8d4c8d0 | 1669 | |
735cc5ca CY |
1670 | When defining a user option using @code{defcustom}, you can set its |
1671 | @code{safe-local-variable} property by adding the arguments | |
1672 | @code{:safe @var{function}} to @code{defcustom} (@pxref{Variable | |
1673 | Definitions}). | |
1674 | ||
b8d4c8d0 GM |
1675 | @defopt safe-local-variable-values |
1676 | This variable provides another way to mark some variable values as | |
1677 | safe. It is a list of cons cells @code{(@var{var} . @var{val})}, | |
1678 | where @var{var} is a variable name and @var{val} is a value which is | |
1679 | safe for that variable. | |
1680 | ||
32770114 | 1681 | When Emacs asks the user whether or not to obey a set of file-local |
b8d4c8d0 GM |
1682 | variable specifications, the user can choose to mark them as safe. |
1683 | Doing so adds those variable/value pairs to | |
1684 | @code{safe-local-variable-values}, and saves it to the user's custom | |
1685 | file. | |
1686 | @end defopt | |
1687 | ||
1688 | @defun safe-local-variable-p sym val | |
1689 | This function returns non-@code{nil} if it is safe to give @var{sym} | |
1690 | the value @var{val}, based on the above criteria. | |
1691 | @end defun | |
1692 | ||
1693 | @c @cindex risky local variable Duplicates risky-local-variable | |
735cc5ca CY |
1694 | Some variables are considered @dfn{risky}. If a variable is risky, |
1695 | it is never entered automatically into | |
1696 | @code{safe-local-variable-values}; Emacs always queries before setting | |
1697 | a risky variable, unless the user explicitly allows a value by | |
1698 | customizing @code{safe-local-variable-values} directly. | |
1699 | ||
1700 | Any variable whose name has a non-@code{nil} | |
1701 | @code{risky-local-variable} property is considered risky. When you | |
1702 | define a user option using @code{defcustom}, you can set its | |
1703 | @code{risky-local-variable} property by adding the arguments | |
1704 | @code{:risky @var{value}} to @code{defcustom} (@pxref{Variable | |
1705 | Definitions}). In addition, any variable whose name ends in any of | |
1706 | @samp{-command}, @samp{-frame-alist}, @samp{-function}, | |
b8d4c8d0 GM |
1707 | @samp{-functions}, @samp{-hook}, @samp{-hooks}, @samp{-form}, |
1708 | @samp{-forms}, @samp{-map}, @samp{-map-alist}, @samp{-mode-alist}, | |
735cc5ca CY |
1709 | @samp{-program}, or @samp{-predicate} is automatically considered |
1710 | risky. The variables @samp{font-lock-keywords}, | |
1711 | @samp{font-lock-keywords} followed by a digit, and | |
1712 | @samp{font-lock-syntactic-keywords} are also considered risky. | |
b8d4c8d0 GM |
1713 | |
1714 | @defun risky-local-variable-p sym | |
1715 | This function returns non-@code{nil} if @var{sym} is a risky variable, | |
1716 | based on the above criteria. | |
1717 | @end defun | |
1718 | ||
b8d4c8d0 GM |
1719 | @defvar ignored-local-variables |
1720 | This variable holds a list of variables that should not be given local | |
1721 | values by files. Any value specified for one of these variables is | |
1722 | completely ignored. | |
1723 | @end defvar | |
1724 | ||
1725 | The @samp{Eval:} ``variable'' is also a potential loophole, so Emacs | |
1726 | normally asks for confirmation before handling it. | |
1727 | ||
1728 | @defopt enable-local-eval | |
1729 | This variable controls processing of @samp{Eval:} in @samp{-*-} lines | |
1730 | or local variables | |
1731 | lists in files being visited. A value of @code{t} means process them | |
1732 | unconditionally; @code{nil} means ignore them; anything else means ask | |
1733 | the user what to do for each file. The default value is @code{maybe}. | |
1734 | @end defopt | |
1735 | ||
1736 | @defopt safe-local-eval-forms | |
1737 | This variable holds a list of expressions that are safe to | |
1738 | evaluate when found in the @samp{Eval:} ``variable'' in a file | |
1739 | local variables list. | |
1740 | @end defopt | |
1741 | ||
1742 | If the expression is a function call and the function has a | |
1743 | @code{safe-local-eval-function} property, the property value | |
1744 | determines whether the expression is safe to evaluate. The property | |
1745 | value can be a predicate to call to test the expression, a list of | |
1746 | such predicates (it's safe if any predicate succeeds), or @code{t} | |
1747 | (always safe provided the arguments are constant). | |
1748 | ||
1749 | Text properties are also potential loopholes, since their values | |
1750 | could include functions to call. So Emacs discards all text | |
32770114 | 1751 | properties from string values specified for file-local variables. |
b8d4c8d0 | 1752 | |
eb22b78c EZ |
1753 | @node Directory Local Variables |
1754 | @section Directory Local Variables | |
1755 | @cindex directory local variables | |
1756 | ||
1757 | A directory can specify local variable values common to all files in | |
1758 | that directory; Emacs uses these to create buffer-local bindings for | |
1759 | those variables in buffers visiting any file in that directory. This | |
1760 | is useful when the files in the directory belong to some @dfn{project} | |
1761 | and therefore share the same local variables. | |
1762 | ||
1763 | There are two different methods for specifying directory local | |
1764 | variables: by putting them in a special file, or by defining a | |
1765 | @dfn{project class} for that directory. | |
1766 | ||
1767 | @defvr Constant dir-locals-file | |
1768 | This constant is the name of the file where Emacs expects to find the | |
1769 | directory-local variables. The name of the file is | |
1770 | @file{.dir-locals.el}@footnote{ | |
1771 | The MS-DOS version of Emacs uses @file{_dir-locals.el} instead, due to | |
1772 | limitations of the DOS filesystems. | |
1773 | }. A file by that name in a directory causes Emacs to apply its | |
6640b281 GM |
1774 | settings to any file in that directory or any of its subdirectories |
1775 | (optionally, you can exclude subdirectories; see below). | |
eb22b78c EZ |
1776 | If some of the subdirectories have their own @file{.dir-locals.el} |
1777 | files, Emacs uses the settings from the deepest file it finds starting | |
1778 | from the file's directory and moving up the directory tree. The file | |
1779 | specifies local variables as a specially formatted list; see | |
1780 | @ref{Directory Variables, , Per-directory Local Variables, emacs, The | |
1781 | GNU Emacs Manual}, for more details. | |
1782 | @end defvr | |
1783 | ||
1784 | @defun hack-dir-local-variables | |
1785 | This function reads the @code{.dir-locals.el} file and stores the | |
1786 | directory-local variables in @code{file-local-variables-alist} that is | |
1787 | local to the buffer visiting any file in the directory, without | |
1788 | applying them. It also stores the directory-local settings in | |
1789 | @code{dir-locals-class-alist}, where it defines a special class for | |
1790 | the directory in which @file{.dir-locals.el} file was found. This | |
1791 | function works by calling @code{dir-locals-set-class-variables} and | |
1792 | @code{dir-locals-set-directory-class}, described below. | |
1793 | @end defun | |
1794 | ||
89bd9ccd CY |
1795 | @defun hack-dir-local-variables-non-file-buffer |
1796 | This function looks for directory-local variables, and immediately | |
1797 | applies them in the current buffer. It is intended to be called in | |
1798 | the mode commands for non-file buffers, such as Dired buffers, to let | |
1799 | them obey directory-local variable settings. For non-file buffers, | |
1800 | Emacs looks for directory-local variables in @code{default-directory} | |
1801 | and its parent directories. | |
1802 | @end defun | |
1803 | ||
eb22b78c EZ |
1804 | @defun dir-locals-set-class-variables class variables |
1805 | This function defines a set of variable settings for the named | |
1806 | @var{class}, which is a symbol. You can later assign the class to one | |
1807 | or more directories, and Emacs will apply those variable settings to | |
1808 | all files in those directories. The list in @var{variables} can be of | |
1809 | one of the two forms: @code{(@var{major-mode} . @var{alist})} or | |
1810 | @code{(@var{directory} . @var{list})}. With the first form, if the | |
1811 | file's buffer turns on a mode that is derived from @var{major-mode}, | |
1812 | then the all the variables in the associated @var{alist} are applied; | |
1813 | @var{alist} should be of the form @code{(@var{name} . @var{value})}. | |
1814 | A special value @code{nil} for @var{major-mode} means the settings are | |
6640b281 GM |
1815 | applicable to any mode. In @var{alist}, you can use a special |
1816 | @var{name}: @code{subdirs}. If the associated value is | |
1817 | @code{nil}, the alist is only applied to files in the relevant | |
1818 | directory, not to those in any subdirectories. | |
eb22b78c EZ |
1819 | |
1820 | With the second form of @var{variables}, if @var{directory} is the | |
1821 | initial substring of the file's directory, then @var{list} is applied | |
1822 | recursively by following the above rules; @var{list} should be of one | |
1823 | of the two forms accepted by this function in @var{variables}. | |
1824 | @end defun | |
1825 | ||
081f7640 | 1826 | @defun dir-locals-set-directory-class directory class &optional mtime |
eb22b78c EZ |
1827 | This function assigns @var{class} to all the files in @code{directory} |
1828 | and its subdirectories. Thereafter, all the variable settings | |
1829 | specified for @var{class} will be applied to any visited file in | |
1830 | @var{directory} and its children. @var{class} must have been already | |
081f7640 GM |
1831 | defined by @code{dir-locals-set-class-variables}. |
1832 | ||
1833 | Emacs uses this function internally when it loads directory variables | |
1834 | from a @code{.dir-locals.el} file. In that case, the optional | |
1835 | argument @var{mtime} holds the file modification time (as returned by | |
1836 | @code{file-attributes}). Emacs uses this time to check stored | |
1837 | local variables are still valid. If you are assigning a class | |
1838 | directly, not via a file, this argument should be @code{nil}. | |
eb22b78c EZ |
1839 | @end defun |
1840 | ||
1841 | @defvar dir-locals-class-alist | |
1842 | This alist holds the class symbols and the associated variable | |
1843 | settings. It is updated by @code{dir-locals-set-class-variables}. | |
1844 | @end defvar | |
1845 | ||
d259fc4b GM |
1846 | @defvar dir-locals-directory-cache |
1847 | This alist holds directory names, their assigned class names, and | |
081f7640 GM |
1848 | modification times of the associated directory local variables file |
1849 | (if there is one). The function @code{dir-locals-set-directory-class} | |
1850 | updates this list. | |
eb22b78c EZ |
1851 | @end defvar |
1852 | ||
b345c561 XF |
1853 | @defvar enable-dir-local-variables |
1854 | If @code{nil}, directory-local variables are ignored. This variable | |
1855 | may be useful for modes that want to ignore directory-locals while | |
1856 | still respecting file-local variables (@pxref{File Local Variables}). | |
1857 | @end defvar | |
1858 | ||
b8d4c8d0 GM |
1859 | @node Variable Aliases |
1860 | @section Variable Aliases | |
1861 | @cindex variable aliases | |
9097ad86 | 1862 | @cindex alias, for variables |
b8d4c8d0 GM |
1863 | |
1864 | It is sometimes useful to make two variables synonyms, so that both | |
1865 | variables always have the same value, and changing either one also | |
1866 | changes the other. Whenever you change the name of a | |
1867 | variable---either because you realize its old name was not well | |
1868 | chosen, or because its meaning has partly changed---it can be useful | |
1869 | to keep the old name as an @emph{alias} of the new one for | |
1870 | compatibility. You can do this with @code{defvaralias}. | |
1871 | ||
1872 | @defun defvaralias new-alias base-variable &optional docstring | |
1873 | This function defines the symbol @var{new-alias} as a variable alias | |
1874 | for symbol @var{base-variable}. This means that retrieving the value | |
1875 | of @var{new-alias} returns the value of @var{base-variable}, and | |
1876 | changing the value of @var{new-alias} changes the value of | |
1877 | @var{base-variable}. The two aliased variable names always share the | |
1878 | same value and the same bindings. | |
1879 | ||
1880 | If the @var{docstring} argument is non-@code{nil}, it specifies the | |
1881 | documentation for @var{new-alias}; otherwise, the alias gets the same | |
1882 | documentation as @var{base-variable} has, if any, unless | |
1883 | @var{base-variable} is itself an alias, in which case @var{new-alias} gets | |
1884 | the documentation of the variable at the end of the chain of aliases. | |
1885 | ||
1886 | This function returns @var{base-variable}. | |
1887 | @end defun | |
1888 | ||
1889 | Variable aliases are convenient for replacing an old name for a | |
1890 | variable with a new name. @code{make-obsolete-variable} declares that | |
1891 | the old name is obsolete and therefore that it may be removed at some | |
1892 | stage in the future. | |
1893 | ||
27d1f87a | 1894 | @defun make-obsolete-variable obsolete-name current-name when &optional access-type |
fc997332 | 1895 | This function makes the byte compiler warn that the variable |
27d1f87a CY |
1896 | @var{obsolete-name} is obsolete. If @var{current-name} is a symbol, |
1897 | it is the variable's new name; then the warning message says to use | |
1898 | @var{current-name} instead of @var{obsolete-name}. If | |
1899 | @var{current-name} is a string, this is the message and there is no | |
1900 | replacement variable. @var{when} should be a string indicating when | |
1901 | the variable was first made obsolete (usually a version number | |
1902 | string). | |
1903 | ||
1904 | The optional argument @var{access-type}, if non-@code{nil}, should | |
1905 | should specify the kind of access that will trigger obsolescence | |
1906 | warnings; it can be either @code{get} or @code{set}. | |
b8d4c8d0 GM |
1907 | @end defun |
1908 | ||
1909 | You can make two variables synonyms and declare one obsolete at the | |
1910 | same time using the macro @code{define-obsolete-variable-alias}. | |
1911 | ||
1912 | @defmac define-obsolete-variable-alias obsolete-name current-name &optional when docstring | |
1913 | This macro marks the variable @var{obsolete-name} as obsolete and also | |
1914 | makes it an alias for the variable @var{current-name}. It is | |
1915 | equivalent to the following: | |
1916 | ||
1917 | @example | |
1918 | (defvaralias @var{obsolete-name} @var{current-name} @var{docstring}) | |
1919 | (make-obsolete-variable @var{obsolete-name} @var{current-name} @var{when}) | |
1920 | @end example | |
1921 | @end defmac | |
1922 | ||
1923 | @defun indirect-variable variable | |
1924 | This function returns the variable at the end of the chain of aliases | |
1925 | of @var{variable}. If @var{variable} is not a symbol, or if @var{variable} is | |
1926 | not defined as an alias, the function returns @var{variable}. | |
1927 | ||
1928 | This function signals a @code{cyclic-variable-indirection} error if | |
1929 | there is a loop in the chain of symbols. | |
1930 | @end defun | |
1931 | ||
1932 | @example | |
1933 | (defvaralias 'foo 'bar) | |
1934 | (indirect-variable 'foo) | |
1935 | @result{} bar | |
1936 | (indirect-variable 'bar) | |
1937 | @result{} bar | |
1938 | (setq bar 2) | |
1939 | bar | |
1940 | @result{} 2 | |
1941 | @group | |
1942 | foo | |
1943 | @result{} 2 | |
1944 | @end group | |
1945 | (setq foo 0) | |
1946 | bar | |
1947 | @result{} 0 | |
1948 | foo | |
1949 | @result{} 0 | |
1950 | @end example | |
1951 | ||
1952 | @node Variables with Restricted Values | |
1953 | @section Variables with Restricted Values | |
1954 | ||
1955 | Ordinary Lisp variables can be assigned any value that is a valid | |
1956 | Lisp object. However, certain Lisp variables are not defined in Lisp, | |
1df7defd | 1957 | but in C@. Most of these variables are defined in the C code using |
b8d4c8d0 GM |
1958 | @code{DEFVAR_LISP}. Like variables defined in Lisp, these can take on |
1959 | any value. However, some variables are defined using | |
1960 | @code{DEFVAR_INT} or @code{DEFVAR_BOOL}. @xref{Defining Lisp | |
1961 | variables in C,, Writing Emacs Primitives}, in particular the | |
1962 | description of functions of the type @code{syms_of_@var{filename}}, | |
1963 | for a brief discussion of the C implementation. | |
1964 | ||
1965 | Variables of type @code{DEFVAR_BOOL} can only take on the values | |
1966 | @code{nil} or @code{t}. Attempting to assign them any other value | |
1967 | will set them to @code{t}: | |
1968 | ||
1969 | @example | |
1970 | (let ((display-hourglass 5)) | |
1971 | display-hourglass) | |
1972 | @result{} t | |
1973 | @end example | |
1974 | ||
1975 | @defvar byte-boolean-vars | |
1976 | This variable holds a list of all variables of type @code{DEFVAR_BOOL}. | |
1977 | @end defvar | |
1978 | ||
1979 | Variables of type @code{DEFVAR_INT} can only take on integer values. | |
1980 | Attempting to assign them any other value will result in an error: | |
1981 | ||
1982 | @example | |
9022ae07 JB |
1983 | (setq undo-limit 1000.0) |
1984 | @error{} Wrong type argument: integerp, 1000.0 | |
b8d4c8d0 | 1985 | @end example |
5887564d | 1986 | |
5887564d GM |
1987 | @node Generalized Variables |
1988 | @section Generalized Variables | |
1989 | ||
1990 | A @dfn{generalized variable} or @dfn{place form} is one of the many places | |
1991 | in Lisp memory where values can be stored. The simplest place form is | |
1992 | a regular Lisp variable. But the @sc{car}s and @sc{cdr}s of lists, elements | |
1993 | of arrays, properties of symbols, and many other locations are also | |
1994 | places where Lisp values are stored. | |
1995 | ||
5887564d GM |
1996 | Generalized variables are analogous to ``lvalues'' in the C |
1997 | language, where @samp{x = a[i]} gets an element from an array | |
1998 | and @samp{a[i] = x} stores an element using the same notation. | |
1999 | Just as certain forms like @code{a[i]} can be lvalues in C, there | |
2000 | is a set of forms that can be generalized variables in Lisp. | |
2001 | ||
ebdbfb95 GM |
2002 | @menu |
2003 | * Setting Generalized Variables:: The @code{setf} macro. | |
2004 | * Adding Generalized Variables:: Defining new @code{setf} forms. | |
2005 | @end menu | |
2006 | ||
2007 | @node Setting Generalized Variables | |
2008 | @subsection The @code{setf} Macro | |
2009 | ||
5887564d GM |
2010 | The @code{setf} macro is the most basic way to operate on generalized |
2011 | variables. The @code{setf} form is like @code{setq}, except that it | |
2012 | accepts arbitrary place forms on the left side rather than just | |
2013 | symbols. For example, @code{(setf (car a) b)} sets the car of | |
2014 | @code{a} to @code{b}, doing the same operation as @code{(setcar a b)}, | |
2015 | but without having to remember two separate functions for setting and | |
2016 | accessing every type of place. | |
2017 | ||
2018 | @defmac setf [place form]@dots{} | |
2019 | This macro evaluates @var{form} and stores it in @var{place}, which | |
2020 | must be a valid generalized variable form. If there are several | |
2021 | @var{place} and @var{form} pairs, the assignments are done sequentially | |
2022 | just as with @code{setq}. @code{setf} returns the value of the last | |
2023 | @var{form}. | |
2024 | @end defmac | |
2025 | ||
2026 | The following Lisp forms will work as generalized variables, and | |
2027 | so may appear in the @var{place} argument of @code{setf}: | |
2028 | ||
2029 | @itemize | |
2030 | @item | |
2031 | A symbol naming a variable. In other words, @code{(setf x y)} is | |
2032 | exactly equivalent to @code{(setq x y)}, and @code{setq} itself is | |
2033 | strictly speaking redundant given that @code{setf} exists. Many | |
2034 | programmers continue to prefer @code{setq} for setting simple | |
2035 | variables, though, purely for stylistic or historical reasons. | |
2036 | The macro @code{(setf x y)} actually expands to @code{(setq x y)}, | |
2037 | so there is no performance penalty for using it in compiled code. | |
2038 | ||
2039 | @item | |
2040 | A call to any of the following standard Lisp functions: | |
2041 | ||
2042 | @smallexample | |
7c08f8ba CY |
2043 | aref cddr symbol-function |
2044 | car elt symbol-plist | |
2045 | caar get symbol-value | |
2046 | cadr gethash | |
2047 | cdr nth | |
1df7defd | 2048 | cdar nthcdr |
5887564d GM |
2049 | @end smallexample |
2050 | ||
2051 | @item | |
7c08f8ba | 2052 | A call to any of the following Emacs-specific functions: |
5887564d GM |
2053 | |
2054 | @smallexample | |
2055 | default-value process-get | |
2056 | frame-parameter process-sentinel | |
2057 | terminal-parameter window-buffer | |
2058 | keymap-parent window-display-table | |
2059 | match-data window-dedicated-p | |
2060 | overlay-get window-hscroll | |
2061 | overlay-start window-parameter | |
2062 | overlay-end window-point | |
2063 | process-buffer window-start | |
2064 | process-filter | |
2065 | @end smallexample | |
2066 | @end itemize | |
2067 | ||
2068 | @noindent | |
7c08f8ba CY |
2069 | @code{setf} signals an error if you pass a @var{place} form that it |
2070 | does not know how to handle. | |
5887564d | 2071 | |
516e1a08 GM |
2072 | @c And for cl-lib's cl-getf. |
2073 | Note that for @code{nthcdr}, the list argument of the function must | |
2074 | itself be a valid @var{place} form. For example, @code{(setf (nthcdr | |
2075 | 0 foo) 7)} will set @code{foo} itself to 7. | |
5887564d GM |
2076 | @c The use of @code{nthcdr} as a @var{place} form is an extension |
2077 | @c to standard Common Lisp. | |
2078 | ||
2079 | @c FIXME I don't think is a particularly good way to do it, | |
5c6ce1c7 | 2080 | @c but these macros are introduced before generalized variables are. |
5887564d GM |
2081 | The macros @code{push} (@pxref{List Variables}) and @code{pop} |
2082 | (@pxref{List Elements}) can manipulate generalized variables, | |
2083 | not just lists. @code{(pop @var{place})} removes and returns the first | |
2084 | element of the list stored in @var{place}. It is analogous to | |
2085 | @code{(prog1 (car @var{place}) (setf @var{place} (cdr @var{place})))}, | |
2086 | except that it takes care to evaluate all subforms only once. | |
2087 | @code{(push @var{x} @var{place})} inserts @var{x} at the front of | |
2088 | the list stored in @var{place}. It is analogous to @code{(setf | |
2089 | @var{place} (cons @var{x} @var{place}))}, except for evaluation of the | |
2090 | subforms. Note that @code{push} and @code{pop} on an @code{nthcdr} | |
2091 | place can be used to insert or delete at any position in a list. | |
2092 | ||
2093 | The @file{cl-lib} library defines various extensions for generalized | |
2094 | variables, including additional @code{setf} places. | |
2095 | @xref{Generalized Variables,,, cl, Common Lisp Extensions}. | |
ebdbfb95 GM |
2096 | |
2097 | ||
2098 | @node Adding Generalized Variables | |
2099 | @subsection Defining new @code{setf} forms | |
2100 | ||
2101 | This section describes how to define new forms that @code{setf} can | |
2102 | operate on. | |
2103 | ||
2104 | @defmac gv-define-simple-setter name setter &optional fix-return | |
2105 | This macro enables you to easily define @code{setf} methods for simple | |
2106 | cases. @var{name} is the name of a function, macro, or special form. | |
2107 | You can use this macro whenever @var{name} has a directly | |
2108 | corresponding @var{setter} function that updates it, e.g., | |
2109 | @code{(gv-define-simple-setter car setcar)}. | |
2110 | ||
2111 | This macro translates a call of the form | |
2112 | ||
2113 | @example | |
2114 | (setf (@var{name} @var{args}@dots{}) @var{value}) | |
2115 | @end example | |
2116 | ||
2117 | into | |
2118 | @example | |
2119 | (@var{setter} @var{args}@dots{} @var{value}) | |
2120 | @end example | |
2121 | ||
2122 | @noindent | |
2123 | Such a @code{setf} call is documented to return @var{value}. This is | |
2124 | no problem with, e.g., @code{car} and @code{setcar}, because | |
2125 | @code{setcar} returns the value that it set. If your @var{setter} | |
2126 | function does not return @var{value}, use a non-@code{nil} value for | |
2127 | the @var{fix-return} argument of @code{gv-define-simple-setter}. This | |
2ee1d59f GM |
2128 | expands into something equivalent to |
2129 | @example | |
2130 | (let ((temp @var{value})) | |
2131 | (@var{setter} @var{args}@dots{} temp) | |
2132 | temp) | |
2133 | @end example | |
2134 | so ensuring that it returns the correct result. | |
ebdbfb95 GM |
2135 | @end defmac |
2136 | ||
2137 | ||
2138 | @defmac gv-define-setter name arglist &rest body | |
2139 | This macro allows for more complex @code{setf} expansions than the | |
2140 | previous form. You may need to use this form, for example, if there | |
2141 | is no simple setter function to call, or if there is one but it | |
2142 | requires different arguments to the place form. | |
2143 | ||
2144 | This macro expands the form | |
2145 | @code{(setf (@var{name} @var{args}@dots{}) @var{value})} by | |
2146 | first binding the @code{setf} argument forms | |
2147 | @code{(@var{value} @var{args}@dots{})} according to @var{arglist}, | |
2148 | and then executing @var{body}. @var{body} should return a Lisp | |
072c7b65 GM |
2149 | form that does the assignment, and finally returns the value that was |
2150 | set. An example of using this macro is: | |
ebdbfb95 GM |
2151 | |
2152 | @example | |
2153 | (gv-define-setter caar (val x) `(setcar (car ,x) ,val)) | |
2154 | @end example | |
2155 | @end defmac | |
2156 | ||
a13e12f9 GM |
2157 | @findex gv-define-expander |
2158 | @findex gv-letplace | |
2159 | @c FIXME? Not sure what or how much to say about these. | |
2160 | @c See cl.texi for an example of using gv-letplace. | |
2161 | For more control over the expansion, see the macro @code{gv-define-expander}. | |
2162 | The macro @code{gv-letplace} can be useful in defining macros that | |
2163 | perform similarly to @code{setf}; for example, the @code{incf} macro | |
2164 | of Common Lisp. Consult the source file @file{gv.el} for more details. | |
651af8c6 GM |
2165 | |
2166 | @cindex CL note---no @code{setf} functions | |
6efddf78 GM |
2167 | @quotation |
2168 | @b{Common Lisp note:} Common Lisp defines another way to specify the | |
2169 | @code{setf} behavior of a function, namely ``@code{setf} functions'', | |
2170 | whose names are lists @code{(setf @var{name})} rather than symbols. | |
2171 | For example, @code{(defun (setf foo) @dots{})} defines the function | |
2172 | that is used when @code{setf} is applied to @code{foo}. Emacs does | |
2173 | not support this. It is a compile-time error to use @code{setf} on a | |
2174 | form that has not already had an appropriate expansion defined. In | |
2175 | Common Lisp, this is not an error since the function @code{(setf | |
2176 | @var{func})} might be defined later. | |
2177 | @end quotation |